I feel some jobs should have a chance for sex or maybe the girl just wants sex so she does it for free. But some jobs won't have sex. One thing I might look into is like an allow sex acts for other jobs restriction so if you check jobs that aren't whores releated wouldn't have sex when those chance pop up but if you allow it then they would have sex.
Glad to hear.

Maybe each girl could have some "horniness" value rise each day. How much depends on a mix of her Libido, maybe certain traits that increase or decrease the rate, and maybe also her job (girls working in the sleazy areas might get turned on a bit by what happens there every day, whereas cleaning or guard duty is not really sexy).
The value could range between 0 and 100 (and some traits could change the minimum or maximum, so girls with certain traits (like Nymphomanic) could always be 20% horny while others could not be into sex all that much) and be the percentage for the check if the girl wants to have sex on her own.
If she gets to have sex of any then, the value is decreased again. The amount of the decrease would depend on other things again: rape would lower it a lot because she doesn't want to have sex after that for a while; sex with the player would decrease little to much depending on whether or not she likes the player. Whores would basically never have this value build up because they have sex every day.
It could maybe also be turned into a handicap for the player if a girl that wants sex really badly will skip her assigned job to satisfy her needs (if the value is really high, she might even ignore the player's ban of sex). The player could counter this effect by taking care of the girls with Interactions or by buying her a chastity belt item, which could also increase Obedience if her built up horniness is high enough.
Just as ideas. I am sure you will take good care of the game either way.
