I'd confront them. Try to talk it out. I wouldn't end things right away, because they might have underlying problems that lead to the cheating. If you really care about them, then try to work it out. If it DOSE happen again... break it off. But chances are... if you find out the reason it happened, it wont happen again. It could be you. Clingyness pushes people away, Jelousy makes people want to rebel. any emotion could trigger a reaction. talk to the person. comunication IS KEY!
As painful as that is mdavu, PLEASE do not take it personally. People with that problem really do not like themselves very much, & feel they have to hurt the other person, before THEY get hurt. It will pass, and good luck with the future..:)
If they cheat then they are no longer giving the exclusive relationship value, they are no longer respecting you, they are no longer honest or trustworthy, and they are placing another persons worth above yours. in other words do you want to continue relating to a person who treats you like your worthless to them, lies to you disrespects you and can't be trusted? If you say no I don't want to relate to a person who would do that or act in that way towards me, then you now know what to do when you find out someone is cheating on you. Walk away ASAP. They do not deserve your time or attention. You can't heal if you don't stop visiting the source of the irritation.