From what I've personally witnessed - cheating is typically done because either (1) the person cheating is being abused and sees no other way to find a fufilling romantic relationship and/or (2) some need is NOT being met in the current relationship (emotional, sexual, romantic, mental, etc.). Cheating is also an EXTREMELY broad category. It isn't just genitals-touching-genitals. It has the potential to be many other things, it all depends on how the individual defines it (for some it's watching porn, for others it's looking at someone in a lustful or loving way)