Congress could enact legislation that would protect the institution of marriage for heterosexual couples by establishing a Civil Union Act that would cover the same 1,100 + rules and regulations that marriage covers. States would be able to defend their historical rights to define marriage, and further define civil unions for those that demanded equal protection given under the law in the institution of marriage.
Gay couples joined by Civil Union would not be unequally locked out of federal benefits given to heterosexual couples. They would however, have to have their Civil Union dissolved in the place of its origin. No different than it is for heterosexually married couples, with the exception of residency requirements which could be handled at a State level.
By Congress enacting Civil Union laws, the gay community would have no cause to demand legal marriage, that affords equal protection of Liberties afforded by the Constitution. They would gain equal protection within the Civil Union Act.
The gay community would naturally fight this because their issue has been legalizing gay marriage, not a gay union. Somehow the gay community believes Marriage would give a legal and social standing to love and commit to their partner, that a Civil Union would not. Anyone that has ever been married, knows that a legal marriage does not atomatically bind a couple to love and commitment, or enhance social standing outside of financial aspects.
Some States have "alienation of affection" that bring third parties into a contested divorce. The third party can be sued for damages when proof is found the legal contract of a marriage was dissolved because of affection of one spouse was alienated due to a third party intervention. Most states have enacted 'no fault' divorce statutes that permit marriage dissolution without fault to either party. As such dissolving Civil Unions would likely be handled in the same manner States handle a dissolution of marriage.
The one thing the gay community hopes to gain by forcing legal marriage over legal civil unions is social acceptance of their morality. Personally, I think the recent Supreme Court decision has backfired. The question of immorality of gay unions is now on the front page. Whereas in the past it was old news, buried somewhere in the lifestyle section of the news.
Time & Tide Changes Everything