It bothers me that this is treated as a Christian issue rather than a some-Christian issue. Guess what, Christians were on both sides of previous marriage issues too.
That gets to the essence of religious liberty. If religious liberty means anything, it's that the state won't always agree with your sect. Even on those matters you consider a threat to that state! (I am reading "Amsterdam: A History of the World's Most Liberal City", by the way, so this is something on my mind what with Spinoza and all that.)
It also bothers me whenever Christians let obsession with sin get in the way of proclaiming the Good News that sin is forgiven. (Not recorded, not obsessed over, not punished by natural disaster, not hair-split by doctrine into tiers, not encoded into civil law, but washed away as if it had never been. Permitted, even, for the Greek word for "forgive" has that connotation as well!)
I have also noticed a small number of critiques of the gay marriage movement coming from the queer community. I can accept their argument that the focus on gay marriage may have attracted attention and support at the expense of other queer causes. I am a little less sanguine about excluding the cisgendered community from those seeking alternatives to marriage (a tradition that extends all the way back to naked Anabaptists parading through the streets of Amsterdam, if not further.)