Popular water-cooler theories:
1) For men, sex and power are very connected. On a caveman, evolutionary level, a key perk of being powerful is having sexual bounty. For women, sex isn’t about power. It’s about (I’ll let my women readers fill in the blank) _________.
2) Women actually care more about their kids. No mother would risk the disruption to the family and kids.
3) Women have been in power for less time and are more conscious of squandering the opportunity.
4) They do have affairs but are better at covering them up.
5) This is a statistical phenomenon. There are more male politicians. When half the Senate is women, half the affairs will involve women senators.
What’s your vote? Other theories?
UPDATE: James Taranto at the Wall Street Journal:

“The real answer is that successful politicians, who usually are middle-aged or older, tend to have the qualities that make men superficially attractive to women (status and power) but to lack those that make women superficially attractive to men (youth and beauty).”