Your question is irrelevant to the gay marriage issue. You don’t have to “prove” you are gay. Two straight men could marry if they wanted to. In fact, if two straight men went to get a marriage license and said they were straight and wanted to marry for benefits only, there would be no stopping them if Prop 8 is not upheld by the Supreme Court. Once “same sex” marriage is legal, the clerk won’t even be able ask you if you are straight or gay. Two guys get a marriage license, it’s legal, and that is that.
That scenario will probably play out a lot as hetero guys who have no interest in marrying/having families start doing this just for benefits; there will be no way for the government to stop it, and the companies will have to provide the benefits (at least for a couple of years, before we’re all covered under Obamacare).
there is no love or mating test in marriage.
If all that is required is benefits, the marriage for immigration papers should be legal too.