Me, too, but ...
The reality is that not only do states use existing marriage status to deny legal benefits to same-sex couples, they also--quite consciously--use existing marriage laws to regulate sexual behavior.
Don't believe me? Just check out the latest brief filed by the State of Michigan in opposition to marriage equality.
One of the paramount purposes of marriage in Michigan--and at least 37 other states that define marriage as a union between a man and a woman--is, and always has been, to regulate sexual relationships between men and women so that the unique procreative capacity of such relationships benefits rather than harms society.And there you have it. The State asserts its power (states cannot have rights) to police your bedroom via marriage legislation.
As such, marriage equality legislation necessarily reduces the State's power "to regulate sexual relationships," and is therefore something which (I suggest) Libertarians should support.