It's rather difficult to second-guess one's motives. I suspect that even those directly involved in the affair could not do it without fabricating rationalizations or things that simply were not there.
Having said that, however, I do not find Clinton's behavior unusual, at least in a statistical sense. I strongly suspect that the great majority of men would do exactly the same thing if they were in Slick Willy's shoes.
That is, they would not resist an affair with a young attractive woman, even though that affair could jeopardize their career.
I think men are generally not very rational when it comes to sex, much less so than women are. It is perhaps the consequence of the differetn ways men and women are brought up in this society. Men are not supposed have any close relationship with other men, and they cannot simply befriend a woman without a sexual innuendo. So they only way they can relate to other people is through sex, and that may explain their irrational behavior as soon as they see any sign of interest or affection shown by a female, especially an attractive one.
I think women are generally more rational in those matters because they are taught how to develop different kinds of relationships, especially with other women, so sex is not the only medium through which they can get close to another human being. Consequently, they are less desperate than men.
So the bottom line is that Clinton's affection to Lewinski was not that unusual. I do not think he explicitly wanted to take advantage of the unequal power relation (i.e sexually harass her), but he was genuinly attracted when she showed interest in him. Most men would feel the same in a similar position. Pathetic, but true. So he who is without sin....
regards,
Wojtek