Let's see... checking the code, this is how a girl gets an STD. If her health is 10 or lower while whoring and she isn't using contraception, there's a 1% chance for each disease that she will catch it. So, there being 3 STDs, that's effectively a 3% chance per sex act.
Of course, what girl are you going to have whoring with 10 or lower health, not to mention no contraception? I guess that makes it almost impossible to get them. We should probably change that at some point, since logically you don't have to be unhealthy to catch an STD, and many forms of contraception actually wouldn't help either.