Just a thought on this - I have recently been reading the Quakers' statements on gay marriage, and was unaware until the other day that they have been performing gay marriages for 20 years now. I know that carries a lot of weight with me, as I have a lot of respect for the Quakers, seeing them as honest, thoughtful and truly godly. Just that one fact may carry a great deal of weight with many Christians, and if you can get people to read their statements on the matter (search Google for "quaker gay marriage" to find documents published by the Society).
What impressed me most was that unlike some resolutions passed by denominations on gay marriage, which you could see as the church bowing to social pressure rather than truly following its convictions (this is what puts a lot of opponents off I think), the Quaker statements make it clear that they reached their position through prayer, consideration and humble dialogue with their gay members, who it seems they never considered excluding. They also passed their resolution before this was really a "hot issue" with most churches, indicating that they were not merely "going with the flow". That could go a long way to making people who have a fixed position on the issue start to really think.
Of course, it will not do much for those whose faith identity is so rooted in their own denomination that they cannot accept wisdom from a group with differing worship practices and organisational philosophy from their own (I'm sure there are Christians out there who consider the Quakers "dodgy"). But that's another argument entirely.