Welcome to our forum.
It's really true that you have to have a foundation of honesty and trust, to build any poly relationship on. There may come a time later on when it's okay to see Rita, but not until Kathy has healed from this past indiscretion and has good reason to know you'll tell her the truth from now on. So concentrate on your marriage right now, and let Rita know this is something you need to do.
I'm sure you'll be able to learn a lot about other poly relationships on these boards. Don't hesitate to air any thoughts, questions, or concerns you may have.
You are in a difficult emotional position and will have to put your own feelings aside for awhile. I know it sucks, but just try to be at peace with it. Your marriage is your #1 priority. Make a resolution to be 100% honest with your wife at all times from now on.
I'm glad you have joined us.
Love means never having to say, "Put down that meat cleaver!"