If you are in a relationship with someone who cheats you have a one sided relationship. Look at it from the other person's side, they are missing something from their life and they are going to get it. People who can have sex with just anyone don't need relationships. Relationships don't matter to them, just self fulfillment. I say, as a person who has been married or 10 years, find someone who is willing to commit to the relationship like you are.
Relationships take time, that's what dating is all about.
You date to find out if this is the person you want to be with. If not, quit while you are ahead, and move on. I think that you should give the relationship a second chance but if it happens again then dump the dweeb. Find your self a person that will value you and there comdiment that they made to you. Hmmmmm, until I were faced with it, not sure.