Hmmm... benefits.
IMHO, the only benefits of making a partnership "official" in the eyes of the state should be:
1. Property. When I pass, my partner (or partners) takes ownership of my property.
2. Medical. In the event I cannot speak for myself, my partner (or partners) should be able to make medical decisions for me.
Off the top of my head... I don't see the legal benefit of marriage other than those listed above. If you think I'm missing something, please point it out.
With the above benefits, I don't see how heterosexuality, homsexuality, monogamy, or polygamy make any difference. It's a social/civil contract. Simple as that.
Again... don't get wrapped around the axle on the terms. As far as I'm concerned, marriage, civil unions, and domestic partnerships should all be one in the same in the eyes of the state.