While there are many, many Christians within the GLBT movement and who are definite “straight allies,” there are a number of very vocal Christians who have made it their mission to discriminate against same-sex marriage. In general, our best advice is not to engage in a religious argument about the issues…we are fighting for civil marriage rights; churches will not be effected by the laws and can still choose whether or not they want to acknowledge a marriage. For example, the Catholic Church chooses not to acknowledge a union between a man and a woman if they are not both Catholic and the law cannot compel them to do so. We recommend you try to drive that point home.
However, we are aware that sometimes such a conversation is unavoidable, so we want to provide you some “talking points.” Stay calm and, if possible, try to come from a loving place. There is a certain beautiful irony in demonstrating Christian love and acceptance in the face of someone who is angrily railing against your support for GLBT rights.
From Queers United: Activist Tip – Reversing Anti-Gay “Christian” Talking Points