Our V just had this same break down the other day. My SOs OSO is a friend of mine that she was interested in, and he had voiced to me many times how pretty she was, so when she came to me to ask if she could get to know him better, I was okay with it. It wasn't the first time I had let her have sex with another man, the biggest thing I had to get over with the last one was how he one night stranded her and was freaking gigantic (below average here, but rock it)! All of us from the first are friends still, so that made it easier. With her current, the her having sex wasn't a big deal. The first time was a little hard, but we agreed that if she told me everything that happened, that it would be okay. Two reasons for this, it's so I know he was doing things that wouldn't make her uncomfortable, and two, it turns her on replaying the whole thing. As the relationship grew, it grew. We all have come closer cause of it, but there have been tough times. Yes mine and my SOs sex life has increased a lot because of the whole relationship, but they have grown closer as well. As we spent time together, J (OSO) and I enjoy teasing our So, and we just had a huge break in the teasing last night. But, before we got there, there was talks about how I was afraid she was going to leave me, tears, we literally were at a rest stop the other night talking everything over because I overheard them talking about how they had sex in the room right next to me that morning. Now, I am okay with them having sex, but they broke 2 rules, no sex while I'm around, and they didn't tell me. On top of that, I had felt like all my attempts at my SO were going nowhere, cause she would shut down on me, and I just learned that he had her going in no time. Discussions were had, apologies were made, and promises for all of us to try harder were put in order. And to me, that is a big thing. It is no longer just you and your SO, it's all of you now. There is a root cause to why he felt the way he did, and it seems like you have found it. But it's not just up to you or him to resolve it, it is up to all parties to fix the issue and come to a compromise that works for everyone. Now, as for the break through we made, J and I could only get so far teasing our SO before she would tell us she was getting uncomfortable. If she tells us to stop, we immediately do, and last night, J and I had her completely naked and aroused on all fronts last night, and had a small showoff competition with our best skills in the process. So, don't give up hope yet, but work on it as a family, not just a singular couple. In our house, the three of us are family.