As a side note - it may be worth reframing ‘the world being largely monogamous and heterosexual’ to something more like ‘societies strongly enforcing monogamy and heterosexuality’.

Anyone who has ever spent any time in a gay club can report that many/most of the other patrons were heterosexual/monogamous too. At least until they entered the club.

Society doesn’t exist for long without a supply of children, and has very strong incentives to enforce certain behaviors.