Finding a romantic partner is perhaps the only thing that can be a real task for every man. And when you find one in the form of a fashion conscious woman, your work is only half done; you have a long, stiff task of maintaining a relationship with her.
We do a lot of stupid things in life and betraying our girlfriend is just one of them. There may be a chance that you've begun liking some other female or you ended up with a hot chick in bed after a party you attended without your girlfriend and now your girlfriend has come to know of it.