It is good to know that you know better than others what violates their conscience or not.
As for drawing the line, firstly I think we should always err on the side of freedom of association rather than on state-enforced compulsion. Just as in a court of law we purport to presume innocence until proven guilty.
Secondly, it is pretty easy to see what constitutes Christianity and what does not. It seems that the teachings of Christ were conveniently written down in a book, and in regards to this particular situation, that book says men should not lay with men. This does not mean that the government gets to interpret what the Bible means for people as some kind of official teaching. Nor does it necessarily mean that anyone can claim that such and such an action is biblical and can’t be challenged. In a reasonable world, I think as long as something can reasonably be shown to be a Christian belief, it should get the benefit of the doubt. For example, “I can’t support same sex weddings” has standing. “I can’t support interracial marriage” does not.