I've tried my hand a making a mod that does just that, but I'm just too inexperienced with programming to even attempt something like that. Basically my idea for the mod would be that you play as one of the three hookers in the game or as any female ped (customizable with Meenyo), location doesn't matter; you stand on the corner/sidewalk and wait for a driver to drive up to you and offer you money for sex. You have to be careful though, because the person could be a cop and take you to jail or he could be a killer and try to kill you after sex. There could be a chance of getting an STD, which would require you to go to the clinic (hospital) to continue working, for a fee.
So if the driver is "legit" he drives to a secluded location where you can negotiate price, once the price is agreed upon, one of the vanilla sex animations play varying in length and depending on the service the client requested. If a cop sees you get in the car he may try to arrest you. Some men may try to "strong arm" your services and get off without paying and you will have have to attack them to get your money or call your Pimp to collect for you. (Spawns a pre-determined Ped that will fight the client for you and pay you a reduced fee if collection is successful).
Well that's my idea!