concurring:
Under challenge in this appeal is the constitutional validity of 12 O.S.1981 § 1289D1 whose provisions make nonmarital cohabita*156tion a ground for downward modification of a decree-imposed alimony award.2 The attack is narrowly anchored on invidious un-derinclusion. Appellant urges that the statute in contest violates the Equal Protection Clause by “penalizing” cohabitation of alimony recipients without subjecting to similar economic disadvantage those alimony obligors who live in the same lifestyle. While I concur in the court’s decision that § 1289D is free from the infirmity sought to be ascribed to its terms, I write separately to add a few observations.
I.
APPELLANT’S CLAIM THAT § 1289D IS INVIDIOUSLY UNDER-INCLUSIVE
The vice of underinclusion — in the Equal Protection sense — stems from the legislative use of an impermissible classification.3 It is not present here.
“A classification ‘must be reasonable, not arbitrary, and must rest upon some ground of difference having a fair and substantial relation to the object of legislation, so that all persons similarly circumstanced shall be treated alike’.”4 The enactment challenged here does not rest on an impermissible classification scheme. It bears a rational relationship to a legitimate state objective of affording alimony obligors a form of relief against economically emancipated recipients living in a nonmarital menage which is akin to that available against remarried recipients. The legislative scheme adopted by § 1289D provides similar treatment for married and unmarried alimony obligees who are similarly situated.5
Unmarried alimony recipients living in a common household with a non-spousal mate as a single economic unit are left by § 1289D in no worse jural or economic position than remarried alimony recipients. Oklahoma law, 12 O.S.1981 § 1289B, makes alimony terminable on remarriage of the recipient unless proper and timely showing is made “that some amount of support is still needed and that circumstances have not rendered [continued] payment ... inequitable.” 6 The test for judicial assessment of a recipient’s post-remarriage support need is strikingly similar to that fashioned for the cohabiting recipient. The latter may also qualify for continued alimony receipt “upon proof ... relating to need for support .... ” 12 O.S.1981 § 1289D.
The states are not restricted by the Federal Constitution from either reducing or terminating an alimony award before its recipient enters into another matrimonial venture that is legally “foolproof”. They are free to decide, based on their own policy, whether an event that falls short of legal remarriage will nonetheless afford a ground for relief from the adjudicated alimony obligation.7 Cohabitation in a non-*157matrimonial setting is usually a reliable external indicator of a newly-formed relationship which — more often than not — spells a change in economic circumstances and hence — for alimony modification purposes— may be treated on a footing similar to remarriage. Cohabitation is not a basis for, and will not justify, modification under § 1289D unless it is accompanied by “changed circumstances” that manifest the presence of economic interdependence within the common non-spousal menage.8 When these elements are present, the relationship is to be viewed as a de facto remarriage.9
Because our statute allows alimony modification not only upon remarriage but also on a showing of obligee’s economic involvement in a home-centered nonmarital union, the Oklahoma obligor is provided with the opportunity for reduction or termination relief against a broader class of recipients— those who either de jure or de facto become economically interdependent with a new co-resident mate.10 The fact that alimony recipients standing in an economic alliance with a nonmarital partner who does not live in a common household were not brought within the vulnerable class of obligees does not make the statutory classification scheme fatally underinelusive. Economic and social legislation with respect to which the legislature has drawn lines in the exercise of its discretion will be upheld if it is reasonable, not arbitrary, and bears a rational relationship to a permissible state object.11 Nor is the challenged statute invidiously discriminatory because it does not permit an increase of alimony against obli-gors who cohabit. Oklahoma prohibits, with even-handed consistency, any post-divorce upward modification of the adjudicated spousal support burden.12 Moreover, punishing cohabitation as a sexual offense — by the sanction of withholding or escalating alimony on moral grounds — cannot be said to be either the intent or the effect of § 1289D. Its terms merely establish a new modification procedure that places the alimony recipient with a de facto spouse in a substantially similar legal position to one who has a de jure mate. The enactment’s focus is neither on the recipient’s nor on the obligor’s personal lifestyle. The court’s inquiry centers on the change, if any, in the recipient’s economic status and on the need for continuing support from the dissolved union’s obligor. Assuming for argument’s sake that any unmarried couple has constitutionally shielded freedom to share living quarters,13 the statute under challenge here is nonetheless free from fundamental infirmity. It does not single out persons living in a common non-spousal menage for a treatment different from that which the law accords to married persons similarly situated. The court’s pronouncement wisely refrains from exploring here the full breadth of protection affordable by *158our fundamental law to the erotic pursuits of unmarried alimony recipients. Inasmuch as § 1289D neither prescribes nor imposes a norm of moral conduct, that question is not before us.14
II.
STANDARDS IN DETERMINING A COHABITING RECIPIENT’S NEED FOR CONTINUED ALIMONY
The court does not address itself to, nor pronounce, the standards trial courts should use in gauging a cohabiting recipient’s need for continued alimony. That issue is not before us. Appellant does not argue that an improper test was in fact applied.
III.
MAY AN ALIMONY AWARD BE MODIFIED BY REDUCTION OR TERMINATION OF INSTALLMENTS THAT ACCRUED BEFORE APPLICATION FOR RELIEF WAS FILED?
Neither does the court’s opinion address itself here to the trial court’s power to effect termination or reduction of alimony payments accrued before the application for relief was filed. That issue must stand unsettled because it was not raised either here or below.15
I am authorized to state that BARNES, C.J., concurs in my views.
. The provisions of § 1289D are:
“The voluntary cohabitation of a former spouse with a member of the opposite sex shall be a ground to modify provisions of a final judgment or order for alimony as support. If voluntary cohabitation is alleged in a motion to modify the payment of support, the court shall have jurisdiction to reduce or terminate support payments upon proof of substantial change of circumstances relating to need for support or ability to support. As used herein, cohabitation shall mean the dwelling together continuously and habitually of a man and a woman who are in a private conjugal relationship not solemnized as a marriage according to law, or not necessarily meeting all the standards of a common-law marriage. * * * ”
[Emphasis added].
. In our statutory and decisional law the canonical term “alimony” is used to denote three distinct classes of monetary awards in matrimonial litigation: (1) support allowance in a decree of separate maintenance, called “alimony without divorce”, 12 O.S.1981 §§ 1275 and 1284; (2) support allowance made while matrimonial contest is in progress, called “alimony pendente lite”, 12 O.S.1981 § 1276; Jones v. Jones, Okl., 612 P.2d 266, 268 [1980]; and (3) support allowance that follows the dissolution of marriage, called “support alimony”, 12 O.S. 1981 §§ 1278 and 1289; see Hughes v. Hughes, Okl., 363 P.2d 155, 157 [1961]. The statute here under challenge deals solely with the third category of alimony.
. Orr v. Orr, 440 U.S. 268, 99 S.Ct. 1102, 59 L.Ed.2d 306 [1979].
. Eisenstadt v. Baird, 405 U.S. 438, 447, 92 S.Ct. 1029, 1035, 31 L.Ed.2d 349 [1972]; Roy-ster Guano Co. v. Virginia, 253 U.S. 412, 415, 40 S.Ct. 560, 561, 64 L.Ed. 989 [1920].
. Disparate treatment for married and unmarried persons who are similarly situated was condemned in Eisenstadt v. Baird, supra note 4, at 1039, and United States Department of Agriculture v. Moreno, 413 U.S. 528, 544, 93 S.Ct. 2821, 2831, 37 L.Ed.2d 782 [1973]; cf. Village of Belle Terre v. Boraas, 416 U.S. 1, 94 S.Ct. 1536, 39 L.Ed.2d 797 [1974].
. Dickason v. Dickason, Okl., 607 P.2d 674 [1980].
. Sutton v. Leib, 342 U.S. 402, 72 S.Ct. 398, 403, 96 L.Ed. 448 [1952], Many states treat an annulment of the alimony recipient’s remarriage as “divorce” and prohibit reinstatement of the terminated alimony obligation of the previous spouse. Hodges v. Hodges, Ariz.App., *157118 Ariz. 572, 578 P.2d 1001, 1005 [1978]; Annot., 45 A.L.R.3d 1033 [1972].
. “Changed circumstances” which manifest the presence of economic interdependence (through the pooling of common resources) may occur within a cohabitational menage in two different situations; (1) when the cohabiting mate does contribute to the alimony recipient’s support or (2) when such mate does not contribute his/her share of expenses. See, e.g., Garlinger v. Garlinger, 137 N.J. 56, 347 A.2d 799 [1975]. The so-called Garlinger test is discussed in The Effect of Third Party Cohabitation on Alimony Payments, 15 Tulsa L. Journal 772, 779 [1980].
. Smith v. Smith, Okl., 652 P.2d 297, 299 [1982] (Opala, J., concurring).
. The familiar tort rule, followed in Oklahoma, that the tortfeasor cannot diminish an injured plaintiffs claim by the amount of loss compensated for a “collateral source” has no application to an adjudicated alimony obligation. See Kimery v. Public Service Company of Oklahoma, Okl., 562 P.2d 858 [1977], Because of their potential for providing a collateral source of support, both remarriage and economic interdependence status with a cohabiting non-spousal mate are explicitly instituted as statutory modification-triggering devices. 12 O.S. 1981 § 1289.
. Village of Belle Terre v. Boraas, supra note 5, at 1540; Reed v. Reed, 404 U.S. 71, 76, 92 S.Ct. 251, 254, 30 L.Ed.2d 225 [1971].
. Hughes v. Hughes, supra note 2.
. See cases cited supra note 5.
. There is no basis in § 1289D — or in any source of legislative history dehors the statutory text — for suggesting that its objective was to impose on unmarried alimony recipients a code of sexual morality after the fashion of the ecclesiastical law’s command clause requiring that following a divorce a mensa et thoro (separate maintenance) the separately maintained wife continues eligible for support (alimony without divorce) dum sola et casta vixerit (so long as she lives alone and remains chaste). See comments in The Effect of Third Party Cohabitation on Alimony Payments, 15 Tulsa L. Journal 772, 776 [1980],
. Oklahoma case law prohibits retroactive modification of child support orders. Reynolds v. Reynolds, 192 Okl. 564, 137 P.2d 914, 916 [1943].