Why do men jump at opportunities to become unfaithful?

I have three guy friends who, on separate occasions, told me bluntly that even ‘faithful’ husbands give in to one-night-stands if given the opportunity (when out with friends, wife is out-of-town, he is on some business trip, etc). “If the girl is physically striking, why not?” When I told them that it may not be the same for some guys, they say that it might be possible but it would be a rare find. So.. I’m now asking to you internet guys. Is this really right? will men ever be faithful at ALL times? completely faithful?