I have heard many explanations why people cheated but one of the most senseless one was: She/He also cheated on me! For me, this is the worst reason that you can mention for anything. If another person is dishonest, why do you become dishonest, too? Just because another person steals or murders, you don’t have to become criminal. Then you are not a tiny bit different, then you cheat just in the same way.
Did it hurt you when you found out that she or he cheated on you? If so, why do you want to do the same thing? Do you think it will give your partner the same pain? Then it is nothing but revenge which is never good. And then also, why would your partner feel hurt like you, if he or she was the first one to be untrue?
And if both of you accept that you are cheating on each other, why do you call yourselves a couple? As I said already, then you are cheating yourself. Then you can go your own ways. Otherwise from that your dishonesty can grow, if you for example try to hide your cheating by lying and if you made one lie you have to cover it with 100 more lies.