In United States v. Windsor, 570 U.S.— (June 26, 2013) the U.S. Supreme Court struck down Section 3 of the Defense of Marriage Act (DOMA) defining “marriage” to mean “only a legal union between one man and one woman as husband and wife” under federal law. (1 U.S.C. §7.)

By treating legal same-sex marriages differently from opposite-sex ones, the court found, “DOMA’s principal effect is to identify a subset of state-sanctioned marriages and make them unequal,” thereby “writ[ing] inequality into the entire United States Code.” Such discrimination violates “the prohibition against denying to any person the equal protection of the laws.”

Dissenting, Chief Justice Roberts noted that “[t]he Court does not have before it, and the logic of its opinion does not decide, the distinct question whether the States, in the exercise of their ‘historic and essential authority to define the marital relation’…may continue to utilize the traditional definition of marriage.” It thus remains unclear whether there is a federal constitutional right to same-sex marriage. Windsor simply compels equal federal treatment of those “lawful marriages” contracted under state law between same-sex spouses. (Slip op. 30, 32.)

Since Windsor, there have been further challenges to restrictions on same-sex marriage. In Whitewood v. Corbett, M.D. Pa. No. 13-1861, filed July 9, 2013, same-sex couples living in Pennsylvania (but married elsewhere) attack that state’s same-sex marriage ban on due process and equal protection grounds. On Oct. 18, the Supreme Court of New Jersey ordered that state to begin issuing same-sex marriage licenses because after Windsor, New Jersey civil union partners were being denied federal benefits as unmarried. Garden State Equality v. Dow.

Windsor does not say whether same-sex unions are now portable from states like New York, which permit them, to states like Texas, which do not. Section 2(a) of DOMA (28 U.S.C. §1738C) purports to relieve states from any obligation to recognize sister-state same-sex unions: “No State…shall be required to give effect to any public act, record, or judicial proceeding of any other State…respecting a relationship between persons of the same sex that is treated as a marriage under the laws of such other State…or a right or claim arising from such relationship.”

Congress enacted DOMA in response to Baehr v. Lewin, 74 Haw. 530 (1993), where a plurality of the Hawaii supreme court held that denying marriage licenses to same-sex couples might violate the equal protection clause of the Hawaii constitution. The House Judiciary Committee predicted that “if Hawaii (or some other State) recognizes same-sex ‘marriages,’ other States that do not permit homosexuals to marry would be confronted with the complicated issue of whether they are nonetheless obligated under the Full Faith and Credit Clause of the United States Constitution to give binding legal effect to such unions.” H. R. Rep. No. 104-664, at 2.

The Windsor court simply noted that “Section 2, which has not been challenged here, allows States to refuse to recognize same-sex marriages performed under the laws of other States.” (Slip. Op. 6.) Yet it is hard to see how §2 could survive Windsor. Sections 2 and 3 are both “directed to a class of persons that the laws of New York, and of 11 other States, have sought to protect.”

Like §3, §2′s “demonstrated purpose is to ensure that if any State decides to recognize same-sex marriages, those unions will be treated as second-class marriages for purposes of federal law”—in this case by depriving them of whatever Full Faith and Credit effect they would otherwise enjoy. Both Sections’ “principal effect is to identify a subset of state-sanctioned marriages and make them unequal.” (Slip op. 20, 26, 29.)

If §2 is also held invalid, what effect will one state be required to accord a same-sex marriage performed in another state? Historically, sister-state marriages have been recognized through comity rather than by Full Faith and Credit. As a result, there is little case law whether the Constitution compels states to recognize each other’s marriages. But now, since the comity one state accords another’s marriages is defined by state law, and some states have already found same-sex marriages repugnant, the Full Faith and Credit issue will be forced. New York same-sex couples moving to Texas will think they are still married; Texas will think they are not.

Although not a “judicial Proceeding,” a state-sanctioned marriage could be considered either a “public Act” or a “Record” within the meaning of the Full Faith and Credit clause. Judgments are accorded the greatest Full Faith and Credit effect (see Baker v. General Motors, 522 US 222, 233 (1998)) including divorce judgments (Williams v. North Carolina, 317 US 287 (1942)), and now same-sex divorce judgments. Logically, other states should recognize the predicate marriages as well. As the court noted in Estin v. Estin, 334 US 541, 546 (1948) “[m]arital status involves the regularity and integrity of the marriage relation…. The State should have the power to guard its interest in [its domiciliaries] by changing or altering their marital status and by protecting them in that changed status throughout the farthest reaches of the nation.” A validly-contracted same-sex marriage should therefore be given effect in other states.

While Windsor fell short of establishing a constitutional right to same-sex marriage, no law that discriminates against sister-state same-sex marriages can withstand Windsor’s equal-protection scrutiny. Section 2 of DOMA will therefore fall with §3, and New York same-sex marriages will be entitled to Full Faith and Credit nationwide.

Robert P. Knapp III is a partner at Mulholland & Knapp. Leah Heifetz is an associate at Kessler Topaz Meltzer & Check in Radnor, Pa.