I mean - bonobos and other apes are very much into female masturbation.

I was prevented from accessing sex education as a teen, and didn't know what the clitoris was - and yet I was able to discover how to masturbate. At 12 years old. So I don't think your claim is correct that the female orgasm is not one of those things that "if nobody told us it existed, our instincts would know that it did" because my instincts absolutely knew what to do and so I learned how to give myself an orgasm despite my religious family's attempts to prevent me learning about sexual pleasure.