If he were simply randy, Cain could have found casual, extramarital sex anywhere, and not just from the prostitutes who, in the mid-1990s, were a regular feature on the DC streets just a few blocks from the National Restaurant Association. But if the allegations are true, what Cain was doing was something different. He was allegedly showering subordinates and an unemployed job-seeker with unwelcome sexual advances that they weren't in a very good position to decline. (It's notable that none of the women who have complained about Cain seem to have outranked him.)
Being harassed at work by a boss or other superior is not the same thing as being on the receiving end of catcalls by construction workers or getting groped in a bar, however unpleasant those things may be. Sexual harassment is illegal because it's a form of discrimination, a way of preventing women from advancing in the workplace. If you don't believe me, just look at what happened to the women Cain allegedly harassed. Sure, they may have gotten some small settlements, but they also had to leave their jobs. Cain, on the other hand, got to stick around, at least for a while, and he has thrived professionally ever since.
If you're a woman, getting sexually harassed at work can ruin your life. When someone in a position like Cain's suggests to a woman looking for a job that the price of his help is a blow job, he's coercing her in a way no drunk bar fly could ever hope to. And she has no good exit. Report the guy and she will forever be branded as a complainer, a career killer if there ever was one. Refuse the sex, the implication goes, and there's no telling what the guy will do to her career out of spite. And the blow job itself, of course, is out of the question (unless the woman is truly desperate, which happens). It's a humiliating and demoralizing situation to be in. Most women who've been there just try to muddle through with as little damage as possible. The few who don't pay a high price for their bravery.