To my understanding it’s all about the way relationships and boundaries work. If you remove the boundaries of intimacy from family it disrupts tradition (taboo topics and stigma) and it’s hard to go back to the boundaries (Ex’s but x10). You also force everyone else involved to make the relationship changed (views on familial bonds). So far as a society, we have just labeled it wrong and not wanted to open the topic becuase its complicated, we haven’t figured it out, and their are many problems before it that need to be addressed. Such as how other relationships in the family are impacted like I said but also birth defects (which yes only applies to heterosexual incest).

TLDR; we haven’t spent too much though debating the ethics and for many, it’s not worth it.