Agree, gorillas usually have quite violent sexual relationships. And many animals unfortunately rape quite often (goats rape other males for 'dominance', dolphins just fucking love rape (victims include relatives and baby dolphins) and ducks have quite a cruel way of going about it where their penis basically gets trapped inside the lady ducks so she can't even try to get away. And god, who knows how often sex between animals is consensual? Lionesses always look pretty pissed off during the act, if you ask me).
So yeah, it pretty much is his 'natural instinct'.