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).
This is the pandora’s box we open when we destroy the conventional institution of marriage. Once you go beyond a man-woman marriage, it is all bets are off.
Gay marriage supporters have anticipated the OP’s question. That is why they are no longer calling it “gay marriage.” They call it “same sex marriage” for a sinister purpose. Taking gay out of it means any persons of the same sex can marry. Gay is irrelevant now. Any man can take any man, get a license, and get married.