Well, for one thing, I never "have" to have sex. If I'm really uncomfortable with someone, out the door they go; if me being uncomfortable is their fault, I still get paid. $250 buys an hour of my time, no more, no less. In general, though, I do have sex with my clients, in part because this is a highly reputation-based business and I like having return clients. For another thing: I really really really like sex. I think the analogy is "Sex is like pizza: even when it's bad, it's still pretty good." Sometimes I've had pretty awesome sex with clients, sometimes it's bad, mostly it's mediocre (but I still get off), and I'd much rather get paid lots of money to have orgasms and make out than not very much money to burn my fingers and roast over a hot grill.
Really, you can't imagine an unpleasant, uncomfortable job? Work fast food for a little while. I had blisters all over my fingers from being burned on the fryers, prickly heat, and a grease smell that wouldn't wash off. Nasty. There are plenty of other unpleasant, uncomfortable jobs, too: coal miners probably get pretty sore (and black lung isn't a picnic), bike messengers ride through bitter cold and pouring rain, retail workers get screamed at, etc..
Let's see, other benefits: I'm not going to lie, it is pretty nice getting complimented by people who are paying you to hang out with them. I get some nice books, too, and get taken to delicious meals. The sex, as I mentioned, is usually at least decent. I meet entertaining people who relax and tell me about their lives, so I learn interesting and sometimes hilarious things. (For example, I had a jewelry-store owner as a client who told stories about wannabe thugs coming in looking for cheap bling and not knowing the difference between CZs and diamonds, heh.)