1) High cost 2) Risk of STDs 3) Ethical qualms 4) Social stigma 5) Money can't (at least directly) buy you lust (this may be the rare rule which admits of no exception).
And actually, I know some (not many) sane (albeit desperate) people who have paid for sex. Dawson probably does, too--it's just not the sort of thing many are willing to volunteer amongst polite company.
I mostly second Dwayne's, Dennis', and Justin's comments on porn. However, I think Dwayne overlooks perhaps the most important factor behind the emergence of "reality" porn. I believe Conan O'Brien once said in an interview that his preferred masturbatory material consists of footage of girlfriends and boyfriends fucking, since there's actually some passion there. The fake tits and absurd sets that were once even more common in mainstream porn serve to highlight the lack of any real passion. Obviously faked orgasms are no more arousing on screen than they are in bed.
Justin's comparison of sexual exploitation with nonsexual forms of exploitation that are arguably just as bad or worse isn't completely compelling. Many forms of alienated labor (e.g. lawerying) wouldn't be all that disagreeable if not for some combination of soul-crushing hours and shit pay. But I suspect turning even just a few tricks a week for lots of dollars would still be pretty bad for most women. (Since Justin shared a bit of well-founded speculation on evolutionarily rooted sex differences, it seems fair to point out that most sociobiologists would probably agree that it's unsurprising that most human sex work is done by women, who are likely generally less psychologically well-suited to the work of fairly frequent and anonymous sex than men would be.)
-- Luke