Usually when I hear of a girl cheating its usually because she feels neglected, abused or her partner isn't putting enough effort into the relationship to 'keep her". Alternatively some women just cheat because they are insecure and need to feel desired by all men. Some women just cheat because they're legitimately dumb. If you are dating someone and have a real connection with them the superficial things... like penis size. Don't matter at all.
And there are other ways to make sure your woman is completely satisfied without using your schlong. ;-P