I've watched cheaters; I've been cheated on and you're right, someone sees, someone knows. Hell, there were times I knew when I was being cheated on and just never said anything...for a while.
I'm crazy good at spotting cheaters. Have I ever told? Once. And I regretted it, I poked my beak in and I was the bad one, at least for the year or so the couple in question stayed together.
As for the men that cheated on me, what I will never get is hy we can't be grown up about this. If you want someone else, tell me, I might cry, but I'll help you pack.