Diagnosing a relationship secondhand is dangerous, so this is just general comments and hopefully something helpful might come of it. Betrayal by others can lead to mistrust in relationships, the important thing for anyone to remember who has suffered betrayal is that it is the nature of that specific person and not that type of relationship, a romantic relationship is not constantly under threat of betrayal that can only carefully be driven off. If one is worried about being hurt again through betrayal, that is not something that can just be ignored, or it will likely be a poison to the relationship, but control of someone who has done nothing to deserve mistrust is a cure worse than the disease. It is a relationship, you can get hurt. If that is too scary or upsetting, one of two things will likely happen: you'll be alone, or wind up with someone devoid of self-esteem willing to be controlled, and the latter only after a lot of doomed relationships before you find your doormat. A romantic relationship is ultimately the greatest trust exercise, that must be handled. The partner's job is to find ways to help reassure him/her that they are trustworthy, not through control, but in other ways, and that depends on the person. A romantic relationship means you have given your heart and body to someone else, if it's a monogamous one than those two things should be unique to that person to avoid betrayal, beyond that are interactions that are not specific to that relationship and should be respected. If that's bothersome, finding a way to get at the root of why that is is key, that person's fear needs to be brought into the light and addressed and the other person's desire to engage in an activity not specific to the relationship likewise needs to be explored, to see why it is important to them. To not address it will allow resentfulness to fester. If it can't be resolved, pretending it doesn't exist won't solve anything.
In my own relationship with my wife, she is aware of my tickling interest and not only tolerates it, she accepts that it's part of who I am. I've been getting some custom tickling videos made, and I know she is completely behind it because: it was all her idea. She has enough trust that I wouldn't betray her that she wants to let me indulge in fantasy because she knows I'll allow her the same freedom, and that at the end of the day the only ones we'll want to share our hearts and bodies with are each other; as my lover, she wants me to be happy, and I her. I know that some are here behind their partner's back, in some cases because the partner dislikes their presence, but what it has done is created the very betrayal the person was fretting over, they have put their partner into a position where they must choose to suppress an important part of themselves or sneak around without their knowledge. Instead, some acceptance is required, and some concern for the worried partner's feelings should be addressed. If they're concerned you're sharing private experiences the two of you have with the world, that's valid; if they're concerned you're spending more time with strangers on a board instead of them, that's valid; if they're concerned that you are engaging in simulated behavior with others the two of you treat as a private thing, that's valid; if they're concerned you want to discover the experiences of others, to discuss ideas related to it, to hear the fantasies of others, those are NOT valid. Of course, there's the reverse, if you want to participate in an adult forum, giving your blessing to your partner looking at adult material they enjoy with a clear conscience is only fair, in fact could be taken as a sign of just how much you trust them and ask only the same in return. Just finding what both can live with, perhaps not discussing TMF is necessary, the partner may prefer to not think about what you're doing here and being reminded of it can feel like rubbing it in their face or opening old wounds.
In short, his feelings are valid, your feelings are valid. Addressing them both is the healthiest way to move forward, to placate doubts without putting unreasonable limits on what's important to you.
