Consent is not treated as a defense theory. Humans are not treated as in a perpetual state of consent for giving away money,
A charity (or even a beggar) rarely needs to prove that the giver gave the money voluntarily.
for being taken strange places by strangers,
A taxi driver (or even bus driver...) rarely needs to prove that their passenger did indeed want to go where they wanted to go... Even if he is a minor (at least for buses). And even if you do accidentally hop on the wrong train, I don't think you've got a case to prosecute the driver for kidnapping...
or any of the other sorts of cases where "consent" defenses are common.... except that they generally are treated as being in a perpetual state of consent for sex.
Really, in daily life, giving or lending money, or giving people a ride is common, without needing to worry about getting a written affidavit that it is indeed voluntarily. And somehow, this still doesn't hamper our ability to prosecute against genuine cases of theft or robbery, or of kidnapping. Why can't sex be treated the same way?