Could they not position themselves as “Christian Business”, having only certain Religious designs?
I think they could, but that wouldn’t prevent a lesbian couple from ordering one of those cakes. They didn’t want to participate in any way in a lesbian wedding.
If it was more, like they didn’t want to put two female figurines on top of the cake, or put pro-lesbian wording on the cake, then your idea would probably work.
If it was total rejection of even selling one of their cakes to be used in such a wedding, then the lesbians might choose a religious themed cake. Then what do you do?
Wonder what would have happened if they spent the entire time talking about Jesus and the sinfulness of lesbianism. Do you think that couple still would have wanted to buy their cakes?