A person cannot love two people at a time but the quest for lustful satisfaction is the basis for cheating. He or she will love one better than the other so the latter is no more love but infatuation.
This quest is driven by several psychological reasons that tend to be either attitudinal or a result of trauma as everyone has a reason or cause for doing something.
Whichever way it is, is immoral. Though there are exceptions to most rules but for cheating, it is damn complicated.