A sin is a sin, regardless. I wouldn’t deny business to gays even if they told me it was for a gay wedding, nor would I deny for the living-together couple. The sin is on them, not me. The only reason they would say anything is to get a reaction. I don’t need to know.
I would just have to change the rules: I wouldn’t put toppers on ANYONE’S wedding cakes/recognition cakes. The fee for set up would be hefty. I would find other ways to help out the non-sinners. ;)
There’s plenty of ways to skin this cat, and you’ve outlined several.