Open Relationship

My partner (fiancé ) and I have been together on 12 years . We have 3 children together a 19 year old (I adopted his daughter) , 5 year old and a 3 year old. We have had our up and downs through out the years. One of the main issues in our relationship has always been communication. I have always expressed my interest in possibly having an open relationship. But he has always said it is something he is not interested in pursuing. Yet throughout many years together I have found out he has been talking to other women. Including having an on and off sexual relationship with one . While all of this I found out before we had kids of our own . We continued to work on communication and agreed on what our limits were in opening up our relationships ( as of 4 years ago) . Recently I found out he has been lying. One of our agreement included us being honest with one another if we asked. I knew 3 months back that he had started his sexual relationship with the same woman as before yet when I asked him he lied to me . It’s not until I confessed to him that I already knew that he finally admitted. One of the things that bothers me is why hide it if obviously have an open relationship for 4 years ? Am I wrong. ? In one of agreements that he wanted for himself. He didn’t wanna know any information of who I was involved with . All of the sudden he is upset about who I am involved with.