Re: social benefits,
I have no problem with that, whether it's inheritance, health, etc.
But you hit a snag as soon as you said:
"Because they've fallen in love with someone, and want to spend
the rest of our lives with that person."
It is not unusual to mistake lust for love. It's part of fallen human nature. As for wanting to "spend their lives with one person", keep in mind that very few homosexual monogamous relationships work that way. Again, homosexuality is not in any way "alternate" to heterosexuality. The physical overrides any true intellectual, not to mention absence, of spiritual relationship.
I am aware that the homosexual lifestyle thrives on danger; in fact, my other gay acquaintance admitted he used to live recklessly, one night stands, picking up strangers in discreet public places...what he didn't tell was how many times he wound up in the hospital, if any. Not sure if lesbians live "that way", but still, the primary focus is on the physical.
But it seems to me that if you're going to call gay marriages, marriage, then it undermines the true meaning of marriage for many people.