True, guys do the exact same thing and in fact have a harder time not doing it. It is in our nature. (ie, marriage, monogamy runs contrary to our nature to survive and sex is the only way to ensure the species survives) A lot of the time one strays because the relationship became stale, your wife/husband is no longer the picture of who you married years before, weight gain, gut from having kids, wrinkles etc. Apparently women like the wrinkles and shit on a man and men can't stand it on women.
And usually it is only one person in the relationship that heads south physically which makes it worse for the one who didn't, especially if the one who didn't is into fitness, etc.