I don't think that the equivalence you imply between forced vaginal or anal penetration and lack of payment is reasonable. In the first instance the nature of the act has not been consented to, where as it has in the latter instance. I.e. the victim in the first instance did not consent to be vaginally or anally penetrated at all. In the latter case the victim did consent to the sexual act(s), but the perpetrator failed to fulfil the terms of their bargain. The legal position in the UK is that in the latter situation the victim's consent was valid because (s)he understood the nature of, and assented to, the act(s) in question. That's not to say that a wrong was not perpetrated, but it doesn't qualify as rape.
The consequences of categorising the latter situation as rape are fairly obvious, I think. If it is accepted that consent can be vitiated by a failure of the other party to fulfil obligations, whether express or implied, then it is difficult to know where the line should be drawn. Should it only apply where there is a contract? If so, why, and what qualifies as a contract (e.g. would the terms of a marriage be relevant?) If non-contractual obligations were to be taken into account, the range of situations where somebody could legitimately claim they were raped are almost endless. Say that my boss strongly implied that I would be promoted if I slept with him. I might have a reasonable expectation of a promotion, but in the event that it was not forthcoming, a claim of rape seems inappropriate. Sexual harassment is more fitting.