It’s like 2 people come into a lawn and garden center and negotiate a price to fix their mowers. Then two friends show up and demand at gunpoint that the proprietor fix their tractors too.
That’s armed robbery and additionally assault if he was brutalized. Is there any doubt that the proprietor would have fixed all 4 tractors if the price was to his liking? Of course not.
If a prostitute chooses to offer a service like sex for money just like any service provider does they need to be held to the same standards of ordinary business practice.
Well then, why limit your view to prostitutes? Let's say that the tractor repairman offers to repair a tractor, and the four men pull a gun on him and sodomize him instead. It's just theft of services because he was offering something for sale, it just wasn't sex though.