without doing a lot of research to come up with a more complete reply, I'll say that the individual Greek city states often had very different taboos and norms, and they changed a lot over time, and if I remember correctly (I haven't read Oedipus in a long time) the issue wasn't incest, specifically, but rather that he committed patricide and then cucked his dad or something? And however folks felt about incest I'm pretty sure Patricide was about as bad a crime as you could commit, up there with desecration of a sacred site, treason against your city, or murdering a guest.

But, again, I'm just going on old memories, so take this all with plenty of salt.