Infidelity can be an attempt to redress the balance in a compromised relationship. In other words, 'If he can then I can.' Some women who have been cheated on decide to give their partner a taste of his own medicine by cheating on him in turn instead of sitting down and discussing whether or not the relationship has a future.
Hell hath no fury...
The bitterest of scorned women might choose who she cheats with on purpose in order to really hurt her partner (the boss he really hates, his best friend or younger brother). Others don't care whose bed they fall into and go for the first available option. In both cases, beware the consequences. Only you know if your relationship is strong enough to start over again after a double dose of infidelity, when the scores are even.