First thing your Str needs to go above 10. 11 would work.
Secondly you'll have to have high enough libido
oh libido, goddammit i didn't factor that at all haha. also don't i need certain amount if agility or perception too? i recall doing a playthrough lacking those two stats and the main character always trips over a bucket instead, when sneaking to rim him lol.
okay so STR 11 and at least 50 libido, thank you :D