Some companies already offer same sex live-in partner benefits (unmarried people who are of the same sex enjoy this benefit while unmarried people who are of complimentary sexes do not).
Some homosexuals have said that they do not fully support same sex marriage being legalized because they feel that there would be a backlash agains the promiscuous lifestyle of some homosexuals. After all, if marriage is suddenly a legal option why are some homosexuals still "slutting" it up? Yes there are heterosexuals who have unmarried relationships and live-in relationships. The homosexual community has expressed some reservation to "settling down" though.
The attack on the institution of marriage is just another attack on the institution of the nuclear family (2 parents of different sexes raising a child).
There was an artilce on FR a few days ago with the headline something about the average relationship (when there is an actual relationship) length between homosexuals is around 1.5 years. And there is overwhelming evidence that even when there is a "monogamous" relationship, especially between men, it almost always includes an understanding that there will be outside sex.
So this whole big deal about marriage is a political ploy for power, it has zilch to do with being married as it has traditionally been understood for thousands of years.