I think cheating is pretty much the be all and end all. But I don't know...if you love them, it can influence your judgement. My friend's bf made out with their mutual friend at a party, then ran back crying and apologised for two days straight and wrote her poems and bought her chocolates and promised he would never hurt her again and said he hated himself...and she took him back. I can't be sure I wouldn't have done the same, but in most cases, cheating = relationship over.