This might not make me popular around here, but here goes:
First, none of this means a church will be forced to hold a gay marriage ceremony.
Second, homosexual behavior is not a crime in this country, so I'm not sure how the government can say homosexuality is ok, but homosexuals cannot get married.
Third, polygamy is a crime in all 50 states in this country, as is bestiality, pedophilia, etc. There are no federal laws against polygamy, so it is possible that it may at some point fall under the same category as Lawrence vs. State of Texas (the anti-sodomy law). Bestiality is not illegal under federal law, but we will not see marriage between humans and animals because of the issue of consent. Pedophilia is against federal law, so that kind of marriage will never become legal.
These are the federal statutes regarding sex crimes.
Personally, I think government should get out of the business of marriage altogether. I don't need the state of North Dakota to approve my marriage to the woman I love. All that matters is our relationship and recognition by my friends and family, most likely through a church wedding. A state marriage certificate is meaningless and is actually a symbol of control. (At times, government has prevented marriage based on race, perceived mental handicap, and even religion.)
As for the legal benefits of marriage: I think a contract could cover those. That is the argument those opposed to gay marriage use to explain why gays don't need marriage, and it is a valid argument.
However, as long as government keeps its snout in marriage, I don't see a constitutional basis for limiting it because of gender.