It's not so much the affairs and flings, but more the Lyin and deceit. Honestly I think it would be ok to do so. having other relationships, sex with others is only ok when the other partner knows about it and is accepts it. Naturally humans are preprogrammed to always find and are attracted to many others and monogamy is not a natural human trait.
The amount of cheating and affairs is at an all time high happening much more then people think even though many would disagree and say "I would NEVER do that!" The deceptions are never acceptable.
If somebody cheated on me by having sex or kissing somebody else I would never forgive them. Only cheaters and whores think cheating is okay. If I were seeing somebody new and found out they cheated in their previous relationships I would drop them immediately
in my opinion, I think cheating is showing how much you really mean to that person. If you really liked someone, you wouldn't want to do anything to hurt that person and cheating hurts. Not saying you're the one cheating! Lol but I wouldn't want to be cheated on so why would I do it to other people?