Good strategy, although I would exchange incest for bestiality (not personally, but for the argument). Gay marriage proponents argue that the activities between consenting adults are nobody's business, and an animal can't really consent but a relative can.
I had a debate with a young woman years ago about gay marriage. She saw nothing wrong with gay marriage, but when asked about incest she exclaimed, "That's gross!!" I then asked her, "Then we should limit marriage only to those you don't find disgusting?" She, of course, had no reply.
You’re correct. Hard to believe she doesn’t find rimming and turd-poking gross.