Clinton's case, the Paula Jones overture was not repeated and the Monica Lewinsky situation was not unwanted. It seems to me if you are uncomfortable with a situation you would stop the situation. It's very personal and very complicated. We're not talking here about getting raped in a dark alley by a stranger at knife point. Usually these cases are between people who know each other and have some kind of relationship and are often between women subordinates and men who are their superiors.
Fear has a lot to do with it.
Shame and embarassment does too. Women are afraid of being punished or humiliated and often they feel guilty because they think others will believe they are responsible for it. Or that they asked for it or flirted or whatever or that they won't be believed. Often women come forward if rumours start flying which are untrue and which hurt their reputation or if they feel that they have ultimately been hurt professionally. Don't ever discount a story because the woman waited.