If you’re at the point where you are considering cheating on your significant other, or feel like you would be capable of doing such a thing, either fix it so you don’t feel the need to cheat or break it off.  It’s obvious you’re not happy, or the person isn’t giving you something you need.  

I don’t understand the point.  I can kind of understand the appeal, the thrill of the forbidden and all that jazz, but if you care at all about other people’s feelings, it’s a bad thing to do.  I can’t see how hard it’d be to break it off with someone and THEN pursue a relationship with the other person.

TL;DR: Don’t do it.