Well... if i were in a serious relationship, i would NEVER cheat. If were to cheat, i wouldnt enter in a serious relationship, for start.
Dont cheating is not SO hard... in fact, unless you be raped, there must be your consent for a cheat happens. So, its simply un-excusable.
So, as i would never cheat, if a girl cheated on me, i would have to sing...
"Hey, Joe..."
or
"I used to love her..."