I'm at a loss. I don't want a divorce...

Background story:

My husband and I have been married almost 9 years. We have a nice house and a beautiful 1 year old boy together. We are overall pretty happy. We have always been very open and honest with each other with everything.

I am also bisexual and would still have urges to be with women, of which he happily indulged me with threesomes or allowing me to have a "friend."

But we are also quite different sexually. He likes bondage and I'm just not that into it. On occasion we have had a female friend come over and indulge him with his bondage fantasies. (All while being totally open with another, setting boundaries, etc)

After having our son we decided that part of our life was over and we want to move on in a strictly monogamous relationship. No extras. Nada.

To today:

Well while waiting for my husband (he is in with the dr) I did a no-no and went through his messenger on his phone.

Turns out he chats with a good handful of women about his bondage fantasies, telling them we are in an open marriage and attempting to have them meet up and asking them to meet me and be my friend. (None of which ever reached out to me) and while the conversation always drops off after his suggestion to meet. It still feels like betrayal. We've always been open and honest and I don't understand why he would lie to me. It just isn't like him.

I don't know what to do. I love him so much. I don't want a divorce. But I don't know how to approach this subject with him AT ALL. ANY advice is appreciated.