Well, I think you first have to be honest with yourself if it is about "retaliation." Because if it is, then that might not be good. I think another thing to check in with yourself about is to see if it is to cover up the painful feelings of your partner being with another woman. That might not be as bad, but I don't know if it serves anyone in the long-run to do things that help avoid understanding and eventually communicating the feelings you have.
Its a whole different thing if the purpose of meeting up with this other guy is to form a genuine connection. And maybe its a mixture ... but I guess here is where you have to check in with yourself, and then with your partner (depending of course on the boundaries that you guys have set up in terms of what communication needs to take place).