The tension is between the harsh and conservative Mosaic Law of the Old Testament and the compassionate and inclusive spirit of the New. If one reads John 8:1-59
1. Jesus went to the Mount of Olives. 2*Early in the morning he came again to the temple. All the people came to him, and he sat down and taught them. 3*The scribes and the Pharisees brought a woman who had been caught in adultery, and placing her in the midst 4*they said to him, “Teacher, this woman has been caught in the act of adultery. 5*Now in the Law Moses commanded us to stone such women. So what do you say?” 6*This they said to test him, that they might have some charge to bring against him. Jesus bent down and wrote with his finger on the ground. 7*And as they continued to ask him, he stood up and said to them, “Let him who is without sin among you be the first to throw a stone at her.” 8*And once more he bent down and wrote on the ground. 9*But when they heard it, they went away one by one, beginning with the older ones, and Jesus was left alone with the woman standing before him. 10*Jesus stood up and said to her, “Woman, where are they? Has no one condemned you?” 11*She said, “No one, Lord.” And Jesus said, “Neither do I condemn you; go, and from now on sin no more.”
So, whilst Jesus forgave the women, He acknowledges adultery as a sin and commands her to sin no more. Whilst Jesus doesn't discuss homosexuality specifically in the New Testament, He does of course cite Genesis and the Law of Moses as authoritative and by extension did, does and will consider same sex marriage as sinful.
This presents a painful dilemma for liberal Christians who wish to extend a hand to people of all persuasions and moreso for homosexual Christian couples who wish to profess their love for one another in front of God. I imagine the latter would do well to consider a ceremony at a Unitarian church although I personally cannot reconcile how a service can endorse a 'sinful' bond; theologically it seems absurd.
I guess the only way to attack this is to imagine being gay and a Christian. That in the context of the Bible it is sinful is indisputable, but we're all sinners. So does one abstain? If one chooses that, the dilemma of gay marriage is removed. If one chooses not to abstain and wishes to formalise one's relationship, but acknowledges the absurdity of asking the church to endorse sin, one ought to marry in a civil ceremony. If one wishes to be married in a church one has to be able to reconcile the conflict between the source text of their faith and the actions they're asking to be sanctioned and whilst I'm unable to see how that's logically possible, I'm willing to listen to reasoning to the contrary.