It's the hormones!

Heaven forbid that maybe the relationship sucks the big weenie.

ABC News: Studies: Women Genetically Programmed to Cheat

"We found that women were most attracted to men other than their primary partner when they were in the high fertility phase of the menstrual cycle," said Dr. Martie Haselton, a UCLA researcher. "That's the day of ovulation and several days beforehand."

A related study, which will be published in Evolution and Human Behavior, finds that women are more likely to fantasize about men other than their mates, but only when they don't consider their mates to be particularly sexy. That UCLA study examined 43 normally ovulating women.

"We're claiming the desire to cheat is what evolved in women, that they may notice they have these desires at a certain point in their cycle," said Elizabeth Pillsworth, co-author of the study and an assistant professor of communication and psychology at UCLA. "Whether they translate into unfaithful behaviors is a matter of their own choosing. Cheating is a choice."

A desire to cheat. Wow! Maybe they made a mistake in choosing their relationship and these are the few days that they wake from the drudgery? Maybe they are just hornier and their mate won't cooperate?

Or better yet, maybe we aren't supposed to be attached to one another for decades at a time.