I have much more respect for men who go to prostitutes than men who lie to women in order to get them into bed. At least the "john" is being honest about what he wants. And wanting "just sex" is a perfectly valid thing to want.
Many women are "whores" in some sense. The man who gets it "for free" isn't usually getting it for free. He has to spend time "wooing" the woman, buying her drinks, buying her dinner, buying a movie, etc, etc. It's far from "free".
The reason men go to prostitutes is to avoid that whole BS ritual and just get to what he wants. It's honest. And there's nothing wrong with it at all.
Many men who visit prostitutes are not unable to get a woman via more acceptable methods, they just choose to forgo that time consuming dishonesty. There aren't that many women that are comfortable enough with their sexuality that they'll go for one night stands, or "just sex" relationships. They exist, of course, but the ratio is not in the man's favor due to social insecurities that women have to deal with.
And, people seem to have a bad view of prostitution and think of it as only the street hookers. Yes, they exist, but they are but a small portion of the sex industry. They're just what people SEE most often due to their visibility and the media, tv shows, and movies. The larger portion of the sex industry are attractive, clean cut women who take care of themselves and are more discriminate about their johns. They don't have sex dozens of times a night, most often just once.
So why go to a prostitute? Because it's easier, and it's honest and straightforward.
Aside from that, there is the fantasy aspect. There are many prostitutes who will be what you want them to be, do what you want them to do... play out just about any sort of fantasy for you. You know... those fantasies that you don't want to tell your wife or girlfriend, or *anyone* else about. They won't judge you, they just oblige you your desires.
