Everyone needs sex, to some degree. Sexual desire is a part of who we are. The need may lessen over time, and may be higher in some than others, but it's there until the day we die.
Prostitutes are another story, and a bit more complex. There's a pretty interesting graphic novel recently released called "Paying For It", by Chester Brown, that explores why Brown winds up frequenting prostitutes, which are legal in his country, and how he drifts into a monogamous relationship with one anyway. I wouldn't say it's an explanation, and Brown doesn't make himself look like a great person, which is a deliberate choice on his part. But it takes a look at what role prostitutes serve in society and isn't judgmental about sex workers, which is welcome.
All that said, cheating on your girlfriend with a hooker means you're a jerk. That, at least, is pretty simple.
I know an asexual bloke. He's lovely. He doesn't need sex or prostitutes. But then again, I don't think many men NEED prostitutes.
Most men need sex. However, some men cannot find sexual partners. Thus, prostitution exists, and will always exist.