You’re saying if i tolerate homosexual relationships, i’m accidentally tolerating homosexual incest adultery?
I don’t care about an incestuous relationship between adults, no, as far as legislating it. Not my business. Obviously there’s a broken promise or two in your worst-case scenario, but should we really make that a problem for society? The courts?
Plenty of people have open marriages and the state doesn’t get involved.
Plenty of people cheat and the state only gets involved in the divorce.
Plenty have incest. If they’re all consenting, it’s their issue.
Guy on my second command married his mother. Not my concern. His father pitched a fit, but that’s their family concerns.
Ultimately, if these two characters are attracted, they’re going to get together, no matter whether most of society approves or doesn’t.