this isn’t actually true.  The federal government requires lots of things to prove that a foreigner wishing to qualify for a spousal visa is in a legitimate marriage, but there is actually no requirement to prove that you’re having sex.  Shared finances, yes.  Shared residence, yes.  Tax returns, yes.  Proof of a commingled life, yes.  Letters of support that you present yourselves as a married couple, yes. Sex tape? No.

(It’s not cynical if you think about it.  It is compatible with a fraudulent marriage that the two people could be copulating like rabbits; but it’s less likely if the two are sharing their money.)

This is not a defense of DOMA; people arguing that gay people could always commit immigration fraud to get a green card are making an exceedingly stupid argument.  Any marriage would have to be entered into for bonafide reasons to qualify for a green card, i.e., excluding immigration benefits.  So the argument has to be that DOMA’s effect on immigration policy is fair, because gay people could always… commit fraud to get around the exclusion.