Pizza guys as fantasy
This trope rarely happens in Real Life, or at least not to the frequency one might think. Part of this has to do with the fact that this client is all a fantasy. In addition, all the pizza delivery services do share a policy of not entering a private residence. This is not to prevent sexual encounters, but to prevent the possibility of the driver being robbed, raped, or kidnapped. It is typical policy to remain at the door, in plain sight of the street at all times and to avoid any darkened areas or locations that look to be uninhabited for the same reason.
Plus, every delivery is timed; the store management knows the exact address the driver is going to, and how long it should take them to drive there, make the delivery and return. Any delay must be explained, and no normal delay is long enough to have a successful sexual encounter even if one were offered. Nor are there an abundance of attractive women so desperate that they will offer themselves to random strangers who knock at their door (though anyone who has been delivering pizzas for long enough has probably been propositioned by a customer at least once).
Also, remember, the pizza guy/gal is working for a business; sex is great, but it won't pay for the overhead costs of running the pizzeria or having a home-delivery service in the first place... so it's not a suitable payment or acceptable substitute for one. Every once in a while, you may get a driver who is dumb enough to sleep with a willing customer, but they will not have a job after (and, if they do manage to avoid getting fired, probably work for a badly-managed store that likely has bigger problems than drivers banging customers).
