I'm going to open the discussion on restarting an old relationship while we both have new relationships
Why do you want to start up your old relationship again?
It's because we're such a good match, maybe we should try it again.
If you start up this relationship again that would be cheating. It is bad for you to cheat on a partner.
It's fine with me. Plus, there is a good chance that my partner is also cheating on me. I'm open to the idea of being polygamous.
While the idea of being with more than one person may appeal to you, if you haven't openly discussed this with your partner then you are being unfaithful in pursuing your ex. Stick true to your commitments or get out of your current relationship.