Hey all, looking for advice. There's a lot to unpack here so sorry for the wall of text.

(BACKSTORY) Wife and I started talking about Lifestyle topics when the pandemic started - we'd kinda danced around it for years talking about fantasies and such, but we actually started really talking about it, listening to podcasts, etc. After many months of lengthy conversations we hit a "eureka moment" where the idea of (both of us) doing sexy things with another woman actually seemed really exciting and we wanted to try and make it happen. We weren't sure about other guys or couples yet, but at the least we wanted to try a FFM.

Well, with the pandemic and all, that wasn't really an easy thing to set up. So, we started talking about couples, and what our boundaries would be in that situation. Initially I was iffy on the idea of seeing her with another guy, but grew to be OK with it. It didn't bring me any particular pleasure, but it also didn't cause negativity or jealousy. And if I got to experience other women at the same time, then great! It seemed like a game changer, where something that routinely plagues relationships (being attracted to and fantasizing about other people) could actually become a shared experience that we could bond over. We set up an online profile and started talking to couples. Haven't played with any of them yet, but definitely have plans with at least one of them to play post-vaccinations, and the 4 of us talk routinely on Kik in a group chat.

So while all that's going on, we discussed the possibility of trying something with one of our single friends - who is male. The idea of MFM was originally one of the last things I wanted to try simply because I wasn't sure how I'd react in that situation, or feel about it afterward. We were also both really wary of trying anything with an established friend. But eventually the novelty of the situation combined with the fact that we couldn't do much of anything else in the LS at the moment led me to agree to it, and we brought it up to him, establishing clear communication, boundaries, etc. before trying anything. He was nervous but agreed, and we moved into it slowly, over the course of several weeks.

At this point we've settled into a bit of a pattern where he comes over and we go on a run, play board/video games, sometimes have sex, and then go back to games. It's been a couple of months and we've played maybe 4-5 times. I feel like I've handled it fine, enjoyed the novelty of it but am starting to get a little bored since this configuration doesn't really do a lot for me. The wife, obviously, has enjoyed quite a bit, and I feel like the friend has also enjoyed. I haven't felt jealousy since I feel we have a very secure relationship and talk about EVERYTHING, and I don't feel like she's keeping things from me. The friend has been very gracious and cool about everything, and hasn't behaved in any suspicious or negative ways. He's very low-drama, which was one of the reasons we felt comfortable asking him in the first place.

(CURRENT SITUATION) So yesterday the wife tells me, in the interest of clear communication, that she's starting to catch feelings for the friend. She calls it a "crush" and hasn't acted on it in any ways that I haven't already seen - i.e. no separate texts, meetings, etc. I trust that she's telling me the truth and I don't feel the need to scour her phone for texts, etc. But, at this point I have watched her have sex with this guy multiple times. (We only do things together.)

Problem is, I'm not taking this well. My last major relationship lasted 10 years and ended in cheating (on her part), and it kinda scarred me mentally. I know this isn't the same situation by any means, but it feels similar and it's almost like I'm going through PTSD. In my mind I'm lashing out, accusing her of manipulating the whole LS thing just to get with this guy, thinking I'm not enough for her, all sorts of terrible negativity. I've been cold and distant with her since yesterday and can't really process any of the constant reassurances she's giving me that our marriage and partnership is her #1 priority. Obviously my reaction is hurting her and I know (in the logical part of my brain) that this isn't really her fault, and she did the right thing by wanting to discuss it with me.

I think the primary issue here is that in my emotional mind, catching feelings for someone else means that your existing relationship has some kind of flaw, missing piece, or chink in the armor, and that it's in dire straits (and based on past experience, doomed to fail). Logically this doesn't make much sense, as we would both agree that our relationship is great and hasn't changed in the last few months at all, but the emotional reaction is there - likely due to past experience which is all I really have.

For her, she views these feelings as a "tiny little box" that have absolutely nothing to do with the state of our marriage or relationship. She's sorry she has them, but they exist and she's not sure what to do with them, so she asked me for help.

I said we would try and determine a path forward regarding the "tiny little box" once I was able to get back into a rational state of mind. I'm trying, but currently I'm not there yet.

So I guess what would help from you all is some data on how often something like this happens (i.e catching feelings for a play partner), and how you manage to deal with it when it does happen. (Can you guess we're an engineer and a scientist?) I imagine this won't be an isolated incident if we decide to continue in the LS and want to be able to handle it better in the future, and help her through it if the situation becomes reversed.

Also, final note - for now we're suspending the sexy parts of our hangouts with the friend, since it seems like the obvious first step.

Thanks in advance.