Guys and girls. You need to stop saying that. No one is asking you to be in a committed relationship and no one wants to hear about how you don't want to be tied down to one girl (or guy) when you've entangled yourself in a monogamous relationship. There are such things as open relationships, friends with benefits, one night stands. You don't HAVE to be monogamous – there are other options! There is absolutely nothing wrong with having more than one consenting partner, with having sexual relations with a consenting friend or stranger. What IS wrong is going about it behind your partners back, without their consent.
I don't get it. If someone who dislikes commitment finds himself /herself in a relationship that demands it, why do they go along with it only to later cheat on their partner? If you're unhappy in a relationship, the solution is simple: get out of it. If your needs aren't being met, if you find yourself struggling, if you feel that you deserve someone better, then grow a spine, let your partner know it's not working out and leave the relationship. Know your priorities: own them and state them – all you have to do is find someone with similar priorities and everything can be hunky dory after that.
Don't want to hurt your partners feelings? Be straight up with them. Don't be a sneaky coward about how you really feel and don't string them along. That's not fair to them and while the truth may hurt, at least you're doing the right thing and you can walk away with some kind of honor.
It's really very simple.