It is common for a person who is already being cheated on by their partner to feel like they have a free pass to cheat also. They think it is not really cheating since their partner is already cheating on them. While it might feel like the fair choice, it is still cheating.
Infidelity in any form is a violation of a couple’s relationship agreement. So, yes your partner may have violated that agreement but do you also want to violate this contract? It is about being a bigger person. That (written or unwritten) contract was created because you and your partner had a connection and while that might be on the rocks currently you have to ask yourself which path you want to take. Would you rather contribute to the dysfunction by committing infidelity yourself, or would you rather take the high road and end the relationship with your partner (or take steps to try to fix things) before pursuing other relations?
Infidelity hurts. It creates wounds in relationships that are difficult to heal. But just because someone hurts you does not give you the right to hurt them in return. It might feel good for a minute, like payback or revenge, but in the end, it will only lead to more pain and suffering long term.
Even though your partner did not respect your relationship agreement, it doesn’t mean that you shouldn’t. Confront your partner. Have a conversation. Seek counseling to determine next steps. But, don’t stoop to the level that got you into this mess, to begin with. Don’t add insult to injury. Just because you may have been the second one to commit the act, doesn’t mean it doesn’t count.