I think in general that if a person is cheating on their significant other it is on the the cheater and not the person they are cheating with. Now I don't think I'd intentionally get involved with someone who I know is in an active relationship, but if I really like the girl I might. I did once, but I was like 17 or 18, and haven't done that again.
That being said, I think too many people get into exclusive relationships when they shouldn't, just because it is what we learn is what you are supposed to do. I think mutually monogamous relationships are good for some people at the right stage of their life. And I think a strong, close, intimate relationship can be formed by two people who aren't monogamous with each other, but only if this is a known behavior, otherwise it'd be "cheating"/betrayal. So I went with the poll option of people should let each other cheat, but that depends on the people and their situation.