"Why is it that when X and Y contract and then X backs off, then if the contract involved sex, Y is guilty of rape, when in every other case X is guilty of fraud?"

Does X get advance valuable consideration for sex?

What is the current legal position on "clipping" - women who pretend to be prostitutes, charge in advance, then don´t have sex and keep the payment?

In any case: sex with minors is illegal on grounds that informed consent by minors is impossible - every reasonable girl informed of the consequences of fornication and loss of chastity would keep her legs together till marriage or age of consent, and every girl who does consent is by that consent proven to have been somehow defrauded about the consequences of sex, so consent is not informed and valid consent either way.

Applying the same logic to property, you could argue that sale of swampland in Florida is inherently illegal - it does not matter if the buyer is not complaining, and it does not even matter if the buyer proves that he actually likes fishing, birdwatching et cetera and bought that swampland at a price reasonable for such use of swampland, knowing he bought exactly swampland.