[Nota Bene: Lest any reader think that the argument below is evidence of what happens when the American School System keeps its charges in the dark about all things sexual, I would kindly tell him to check out the New Hampshire Case Blanchflower v. Blanchflower (click on the link, please; I'm too lazy to summarize the facts of the case here). The argument I give below was not taken from NOM, any other SLPC-designated hate group, or any seemingly sexually unenlightened group like, say, the Catholic Church. Rather, the argument is pretty much the same one which was given by the Liberal (and, therefore, enlightened) Majority. The knuckle-dragging Conservatives dissented! Furthermore, the Liberal Majority Opinion was a victory for a Lesbian, who was, as co-respondent in the original divorce case, one of the plaintiffs.]
The legal recognition of same-sex "marriage" has nothing to do with the societal approval of whatever same-sex couples may do to express physical intimacy because the re-definition of marriage required to accommodate same-sex couples necessitates a re-definition of what constitutes marital consummation.
But no concept of marital consummation will cover both opposite-sex and same-sex couples. The English Parliament tried to do so when it legally recognized same-sex "marriage" and failed. It just cannot be done.
If you define marital consummation as coitus, then you exclude all same-sex couples, and that's just heterosexist bigotry. But if you define marital consummation more broadly to include any penetration, then you've still excluded Lesbians who eschew strap-ons and fisting and gay men who prefer frottage.
And even if you re-define it to mean anything that leads to an orgasm, well, then you’ve just made any legally coherent definition of marital consummation impossible. Given that there are as many kinks as there are fantasies, it would be impossible for the law to acknowledge every single potentially orgasmic act. What? Two dacryphiliacs request an annulment because of Sjögren's Syndrome? Can an autagonistophiliac marriage be annulled because of stage fright?
In short, what constitutes consummation becomes hopelessly subjective, thereby making it impossible for the law to acknowledge any objective or coherent concept of marital consummation. Thus, the law will have to jettison it altogether and therewith the idea that marriage has anything at all to do with sexual activity or physical intimacy of any sort. If "marriage" then has nothing to do with sexual relations or physical intimacy, then it cannot be said that "marriage" constitutes societal approval of any physical relationship, be it heterosexual or homosexual.
"Marriage" becomes no more than a friendship, and if the state still wants to encourage that these friendships be exclusive by means of various sanctions for adultery--be they criminal penalties, as is still the case in the military, or simply possible grounds for a fault-based divorce--then obviously adultery must be re-defined. It can no longer be understood as extra-marital sexual relations because "marriage" no longer is a sexual relationship. It is, to repeat, simply a friendship, and, therefore, "adultery" would have to be extra-marital friendship.
Same-sex "marriage" is a patent absurdity.