I think it's a gap in the law. I don't think it qualifies as rape, on balance. To some, rape involves force, violence, or threat of violence - fear - which is absent. To most, rape involves loss of self-determination over something intimate, which is present but qualified: It's retrospective, and could be fixed by a modest sum of money.

At the same time, I think it's far more morally reprehensible than mere theft because the sex worker has made an extraordinary personal concession by giving sex, and so not paying is demeaning and damaging. There ought to be a special legal concept for this, but it's tricky because (I feel) sex should not be an enforceable part of a contract. Some kind of "breach of intimate agreement" concept might work.