Let's replace 'college women' with 'software engineers.' Where's the difference? You need money, you have a skill set, you trade your skills for money. Is that not exploitation?
How about at the "lower" end of the wage pool? Are employers not "exploiting" teenagers with "financial need" by paying the teens minimum wage?
Lest you reply "but ANYONE can have sex," allow me to suggest that you might not have the ability to offer me sex in a manner for which I am willing to pay. I don't get this taboo against paying for sex.
EDIT: I am aware that a thread on this topic appeared while I was typing. It happens.