Excellent question!
While I don't believe a same sex unions should ever be called "marriage"...because by definition I don't believe they are or will ever be true marriage....I am a not so sure about the question of benefits to "domestic partners".
For example..I think it is awful to ban gays from visiting their sick partners in the hospital under rules that only allow heterosexual spouses in.
I mean in who's interest is such a rule? Is this the proper situation to punish people for not being the traditional hetersexual couple? When the closest person to them might be dying?
I think such rules can be simply hurtful and help nobody in any way.
Same thing with some other benefits.
What are we losing if we extend some insurance benefits to gay partners that are commonly given to heterosexual spouses?
We're not paying anymore than we would if they had a hetero relationship...what is the point of punishing them because we don't agree with gay relationships?
Is this the way to express our moral convictions..to make them suffer...while we will agree to the same benefits for heterosexuals who might be cheating on each other...married in name only etc.
Anyway...your question is a good one. And there is no easy answer.