What, exactly, are her beliefs?
She's ok with the adultery, murder, backstabbing, gossip, sex outside of marriage, theft, alcohol and drug abuse portrayed on soap operas, but not gay people?
She'd like to pick and choose which sins she plays a part in glorifying.
Interesting.
Of course. All the things you list may happen on the show, but her character is not required to approve of them.
They are asking her character to approve of something she doesn't approve of, so she has drawn the line and said no.
Where ever you choose to draw the line in your profession, there are going to be people who think you waited too long to object, or you're not a team player, or you were too quick to object, or because you didn't object before you have no right to object now.
In fact thats one of the most effective hammers they hold over people, the idea that since you are flawed you have no right to uphold moral values, or the idea that since you gave in on something else you have no right to object now. But it always happens that way; most people will hesitate before finally digging in their heals. And most people have mistakes in their own personal history. And especially if there is a price to be paid, most people will try to find a way to coexist for a while before finally saying, this is too much. No.
Probably it would be different if she was playing a villainess, but she evidently plays someone who is considered moral. So she didn't want to play a moral character approving of what she considers immoral. That was her red line.