When you masturbate on to a female's belly, and after cumming, she scoops up the semen and inserts it into her womb, thus impregnation.
You're pollinating her, like a flower.
When you grow tired of missionary
all the time, you have to freshen things up with bizarre / absurd
sexual acts, that somehow turn you on more than normal sex.
Masturbating in front of the female, is like when you fap to hentai / porn, which is part of the appeal.