I had sex with my brother but I don’t feel guilty

The whole story is pretty interesting, but it’s this somewhat-throwaway passage that caught me.

I think incest is traditionally seen as bad, but in some cultures that
isnt the case. When I was small I asked a Sunday school teacher if Adam and
Eves children married each other since they were the first people on earth.
She just laughed and didnt reply. Having children with Daniel was never an
issue and we were always careful about contraception.

She’s lucky she didn’t go to one of those churches where they punish you for those sorts of questions.

In Time Enough For Love, Heinlein addresses the whole issue of sibling procreation. Apparently it’s all about shared genes or somesuch. (Yes, I know, it’s sci-fi.)