I married my wife 20 years ago. We have been and are in passionate love since then. Back then already, she was so hot in my eyes and she still is. We both have had quiet a number of sexual partners before marriage. After marriage, it's just the 2 of us enjoying the sex.

We have told each other almost about every sexual experience in our past. (I say "almost" because understandably some details aren't important or relevant anymore).

A few years into our marriage, I started to fantasize about sharing her in an MFM (with me as the other male). These fantasies made me go crazy and made me even more attracked to her. My sexual appetite for her become more lustful. Then I started to enjoy thinking about her past when she was having sex with her boyfriends. After that, I was showered with fantasies of FFM in which I liked to see her and another woman perform cunnilingus on each other.

I have never fantasized, dreamt or thought of being a cuck. I hate it and I would never let that happen. This I know about myself.

Of course, I have never kept this a secret from her and have told her (with some limitations) about my fantasies. I have told her that I'd love to see her in an MFM or FFM. In response, she has also jokingly/or sometimes seriously told me that she'd enjoy them too. For example, she has told me several times that she'd like to see me perform cunnilingus on another woman or that she'd like to see/be in a gangbang :)

My point/question here is:

Realizing an MFM or FFM is in and of itself another topic and issue, which I don't want to discuss here. Though if anyone wants to comment on it, they are more than welcome.

I have tried to research this "phenomenon", but haven't reached any hard conclusion about what it is psychologically, mentally and emotionally that drives these thoughts/ideas/fantasies in my brain.

Can anyone help me understand the science behind this please?