Just to clarify my views... I am not a supporter of gay marriage. I am a supporter of equality. I think the government should get out of the marriage business altogether and give it back over to the religious institutions.
For legal purposes, I think the government should offer civil unions to 2 or more consenting adults, which is really just a bundle of contracts and legal agreements. I don't see any reason to restrict these contracts to opposite sex pairs, or even to pairs at all. If 3 or 4 people want to enter into one of these contracts, so be it, let them.