Having Congress force a church to do anything. Just as a church is not supposed to try to force Congress to do anything.
Not always several churches my more Fundamentalist family members belong to are not tax exempt, they do so with their schools and charitable operations but no the church proper. So they can speak out in the pulpit against political parties or anything else they wish and be politically active.
I argue with the people that feel restrained by the laws just don't be tax exempt then the government can't make you do anything. Just pay fair taxes.
On this issue to get back to it it doesn't matter if you make all current "marriages" civil unions the Churches will not support it its still to them an attack on the traditional nuclear family. I brought this up with a relative that is in the American Family Association (down her in Florida a major religious evangelical group) and they oppose anything that would justify gay unions if that replaced traditional marriage. Plus in Florida it can't be done our Amendment 2 seems to ban those.
Its better to have something less for non-opposite sex couples and those that do not wish to be married but offers a reasonable number of basic rights that everyone can agree on. Then maybe you can get the conservative Christians and the others on board.