I'm having a bit of trouble finding the argument here. Let's take a "transaction" that most of us think is just fine: accepting a proposal of marriage. If Pat agrees to marry Robin and then gets cold feet, Robin can't force the issue. But what of it?
Or take another example: I agree to buy your house. I sign the contract. And then I back out. In most jurisdictions, far as I know, you can't sue me for specific performance; you can't force me to buy the house, though there are various damages that you would be entitled to recover from me.
As things stand in most places, a contract for an act of prostitution isn't enforceable, and so Travis has no legal claim against Elise -- particularly if she gives back the money. But suppose that these sorts contracts were legal, since your issue is presumably with people who think they should be. In that case, there's still no reason to think that Travis has some sort of right to rape Elise, though depending on the legal regime, he might have a civil claim against her that would allow him to recover monetary damages.
In fact, there's a strong whiff of red herring here. All of us agree that some kinds of consensual arrangements are legitimate, and the law recognizes a good many. That means we'll always face the question of what someone is entitled to if the other party reneges on a legitimate agreement. The answer will depend on the case. It might be nothing at all; not all private agreements amount to enforceable contracts. It may be that some sort of monetary damages are in order. It might be, depending on the case, that requiring the original agreement to be kept is the remedy. It all depends. But the fact that people sometimes go back on consensual agreements tells us nothing at all about whether private acts between consenting individuals should always be permitted. In particular, someone who thinks prostitution should be legal doesn't need to be committed to the bizarre view that a prostitute who has a change of mind should be required to submit to rape.