First I need to provide some context.

Before meeting Monica, she used to date a mutual friend, Paul. Nothing official, just one or two dates but I know a lot of chemistry was going on there because of what she told me. When I met her we used to date, but nothing official as well. I saw other girls and she also had social her social life. Things changed and after almost a year (I can't remember well) she was practically living with me.

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I stopped seeing other people and I thought she had done the same but we never talked about it. One day we were kissing and noticed a hickey. I asked what it was and she naturally replied that it was Paul. I was angry, but since nothing was official I simply said that I felt that our relationship was serious given the time and circumstances and that I understood that I had no right to demand anything, but I would not go ahead if she saw other people. She agreed and that was it.

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Now comes the part that bothers me. Two years have passed and today Paul is my coworker and friend. In November he left an abusive relationship and his life and health came down. We did our best to help him. A couple of months ago we were buying new clothes for him. Monica saw a dress and asked if he would date her in that dress. At the moment I said nothing but this left me deeply disappointed, with anger and jealousy. When we got home I talked to her and said I was uncomfortable with her asking Paul if he would date her. She angrily said it was just a conversation, turned around and fell asleep. I do not know if she got angry because I felt jealous, for confronting her or the way I said it, but I did not want to insist.

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Since then I have noticed that she tries to boost his self-esteem with compliments I have not received in a long time. Almost every day we go to his house as soon as I get home from work and stay up late and she does not seem to want to leave. We do not have time for us because she does not want to.

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This has made me reevaluate our relationship. 

Am I the asshole of being jealous, even though his life is a mess? Was I the asshole for saying I was uncomfortable?