Just from Anthro 100 (which I took a couple years ago), I can throw in that for starters, the biological issue isn't the only problem with incest. There's social stability issues to consider. Much as we like to look and think about human monogamy, the truth is one-mate-for-life isn't the actual pattern we really see. It's more often serial monogamy: one partner at a time. Which is to say: LOTS of romantic relationships break up. Now, yeah, there's good breakups and bad breakups, but the "good" ones are really really difficult and not entirely commonly pulled off. All of which is to say: romantic relationships within a family unit that's supposed to serve as a stable base of support have the potential to upset said stable base.
Furthermore, there's much more potential for types of unhealthy relationships that DO exist outside of incest, but not as often. Existing power dynamics, scripts, pre-established roles can all contribute to relationships that are potentially coercive or abusive. And even if both parties don't want that to be the case, it's kind of the way we're wired that that stuff would affect people.
That's all I got for now.
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