Only women believe that they can be "just friends" with men. Guys know that on any given day a woman may be lonely, depressed, drunk, just broken up with a boyfriend, or in some other way vulnerable and craving intimacy. This is why the smart guys cultivate LOTS of female friends. They are supportive of them, never pressure them, always tell them the truth, and make themselves generally available. Women who would never dream of sleeping with their "buddy" can turn on a dime and drag him to bed at a moment's notice. These male "friends" are simply jackals slinking around the edge of the herd waiting for a weak moment. An intelligent man will NEVER trust the male "friends" of a woman they are involved with. Most of them have done the same thing and know the motivations of these opportunists...

[ Only shallow men can not just be friends with women, I have Always seemed to have more male friends all my life.....don't know why. ]

For the same reason cats hang around fish markets.
Anything can happen..

Yep. Even if it's all innocent, depending on someone else for emotional support instead of biting the bullet and working out your problems with your S/O is it's own form of cheating.
It shouldn't be surprising that the sympathetic ear someone runs to, will often as not turn into a bed partner..