Let me just state that this post is my OPINION and is not fact.
I believe any girl will cheat on a man provided thet 1) a 'better' man comes along and 2) the girl can justify cheating, "He seduced me/I was drunk/etc"
By better I mean, better looking, better 'game' and just generally better.
Take this summer for example, I was in the USA, and the accent gives me the advantage over most American guys there...plus the fact that I was a soccer coach at the time, plus I'm tall and at least decent looking. 3 of the girls I got with all told me afterwarda they had b/fs, one even talked to him on the phone straight after...and these were all regular "good" girls.
Is this good news or bad news? Well, if you have a g/f and are rubbish in bed, boring, ugly etc. its bad. But there are ways you can stop this happening, you just have to be 'better' than any competitors, that means keeping her on her toes, having excellent sex, great conversation, funny, exciting, etc.
On a last note, you really can't blame a girl for this behaviour...they're hardwired to select the best possible mate....just like guys are.
PS; just cos a girl says she won't cheat doesn't mean its true. I've seen some "player" types get with girls who have used this exact phrase with conviction. Girls/women generally act upon their emotions as oppsed to men who act upon logic.