Edit: Actually I retract this comment. You bring an interesting point, and shouldn't be downvoted. I guess the argument "to prevent abuse" includes "to prevent people from exchanging their lives for money". And you could say the same for sex - people should be able to choose to have sex / be murdered / donate their organs, but shouldn't be able to exchange that for money.

I guess the counterargument would be, that the exact definition of "exchange" depends on whether you're actually giving something - arguably, you're giving up your life / kidney (and most people aren't willing to give that up, regardless of whether they're being paid or not), but not necessarily giving up sex (and many people are giving it up pretty often... the only difference is, are they also getting paid for it, or not).