Our marital arrangement
When I was a teenager, I was in an intense romantic and sexual relationship that left a big mark on me. I was so in love with the person that I didn't care it was a "forbidden" relationship. It was an incestuous romance between a cousin and me, and when she died in an accident I was devastated. I managed to move on for the most part, but for literal decades I never had such intense feelings for any other woman, so dating eventually became a stale affair to me. I reached my mid-30s unmarried and feeling kind of indifferent towards relationships.
Then I met the woman that ended up becoming my wife.
She was 24, 10 years younger than me then. We started dating and fell head over heels for each other pretty quickly. I thought "this is it", she made me feel things I hadn't felt in a very long time. And I got along great with her family. Her older brother seemed to love her very much, and was very welcoming. At least at first.
After a couple of years dating, when things started to turn serious, she became distant little by little. We knew each other very well by then and were very open about everything, so I asked her what was up.
Her confession was quite something, and it floored me at first. She told me she was confused, because I had changed everything for her. She told me a story: since she was very young, her usual routine was to have a revolving door of boyfriends to "keep appearances", and it was normal for her to date them for a maximum of a year before breaking up with them. The truth was that, since they were little, her brother and her had been in love, a relationship that turned sexual as they became teenagers. They had kept it a secret to everyone by periodically committing to different partners (they consciously chose to not to cheat on them), and being a couple in the dark for longer periods of time. Thing was, I had changed that, because she was deeply in love with me, and felt torn between me and her brother.
It was a shock at first, but I could understand her, so I actually ended up telling her my story (she knew I had a very intense relationship in my past, she just didn't know I was related to the person). Open-minded as we are, we continued to accept each other and come to a new arrangement. After talking with her brother, we started a polyamorous relationship... and we felt relieved and freer than ever before. And so began my experience with group sex, as well. The three of us are heterosexual, so no "crossing swords" or anything, but we make love to her simultaneously a lot of the time since then, and let's just say she's never been more satisfied.
This arrangement carried over to our marriage. On the surface, I married her like a normal couple would do, but he continued to be her lover. After quitting his job, he told everyone he had been fired and had trouble finding work since, so he moved in to the guest room over the garage in our house. Truth is that was rarely used, as we shared the same bed for many of the early years of our marriage.
That had to change when my wife gave birth to our first baby, since it would have been very hard to explain why mommy, daddy and uncle slept in the same bed. He does sleep in the guest room now but remains her commited lover, as she expects our second baby together, ten years after we met.