that's actually exactly what a storyteller should be doing, stopping mc from getting his way. that's what creates the 'engaging characters' people always claim they want. where as mc getting his way produces those flat characters and superficial stories people claim they hate.it's nothing more than punishing the player because he played as intended (he made the MC have a lot of sex).
but yeah, I can't really see it working with std. even in the brothel scenario pregnancy can do all the same things mechanically, with far greater emotional potential for both good and evil.