These people are not Christians, their Nazi's. Jesus hung out with hookers, gays & misfits. He didn't give a rats ass who or what you were.
I would serve the adulterer though, I would not create a cake to knowingly honor such a relationship. then again that's just because I'm against affairs as destroying families, not so much the religious aspect.
but I'm sure if the "Aldulterer" went to a Christian bakery and said I want a cake for my secret lover for when I tell my wife i'm working late, they would object to that to.
Oh, and that Christian who helped me was gay.
I wonder if they would serve a Petafile Priest.
My personal feelings is that there is a stronger case to be made against such discrimination based on the notion that it violates the religious principles of tolerance and mercy when people are eager to condemn people in this way. If you really analyze things strictly no one perfectly follows religious principles. As such we all are full of sinful tendencies and therefore we need to practice tolerance, mercy and try to show love to one another.
As far as rights go, it seems to me like you may be discriminating against a person by denying them service at your place of business because you think they have some fault with respect to religious principles, but on the other hand you are violating a persons rights to practice their religion as they see fit if you make them serve such a person, no matter how misguided their understanding of religion might be.
If my post offends you, I deeply Apple-O-Jize.