Thank you all for reading and responding.
Our sex life has slowly diminished by my wife having severe back trouble for several months, which turned out to be a hernia. More or less at the same time she was working through some very emotional issues dealing with sexual abuse in her childhood. There was nothing wrong between us, at least not that I have been aware of. But now I feel left out, because I have given her all the time she needed to deal with her issues, and now she started having a sex life with someone which is not me. That hurts. And it doesn't make me a more secure or attractive person, I can see that.
RP, that's good advice, yes (but exactly the thing that hasn't happened). My wife has always argued vehemently against having multiple relationships, casual sex with others, or anything else outside the realm of monogamy. Now we have gone from a monogamous marriage to a full V, where she sort of demands the right to be with her boyfriend twice a week, sleep over and have sex. This all in about four months.
I know I am partly responsible for this. In the beginning, I have encouraged her to develop a friendship with her now-bf. But things progressed. She has repeatedly said she needed to be closer with him, or else she would feel controlled by me. I have, several times, said things were okay, and we would move the boundaries a great deal. She now accuses me of coming back on my word (which I do, that's true.) However, I have also have said on many occasions that things were going too fast, that I didn't feel happy or comfortable, that I needed more time. And these things haven't been heard or at least they haven't been acted upon. Not to mention the emotional upheaval, bouts of crying, arguments, which I suspect have only driven her towards her boyfriend more. Once, after a particular emotional crisis, I tried setting a strict boundary (no sexual intimacy for two weeks) but that turned out to be not acceptable nor possible for her, because she felt I was controlling her life. So I (again) didn't stand up for myself, capitulated fully, and said she could do whatever she wanted and that I would just have to cope. That was the only boundary (e.g. no boundary) she accepted at that time. I have much to learn in that respect.
So, in the end, I really feel all her attention has been focused on her relationship with her bf. And that is also the predator I have some deeply rooted issues with. Not a good combo. It still happens that almost every discussion we have revolves around her being able to see her bf. Even our therapist has seen that. It's not: 'How much time do we need to address the issues we have in our marriage', no, it is 'how much time do you need before I can have my relationship with J the way I want it'.
Maybe this is the way NRE affects her, maybe she is being selfish, maybe I am not as big a person as I would like to be. I don't know, it's all very complicated..