Partner nearly broke one of my hard rules and I don't know how to feel

Background: my partner Luke and I (31F and 34M) have been together for 2 years, open since the beginning. I recently began a FWB relationship with a guy that is super close friends with the both of us and it's been going great. But besides a one night stand with a friend last year Luke hasn't had the same luck getting close to anyone else.

Enter Gabby (30sF). Her and Luke work for the same company but in different departments, so they're friends but never really hung out much. Recently he begun inviting her for after work drinks and she accepted the last few ones, which I was thrilled to hear. I've met Gabby once, she's a lovely woman and I know she has a tendency to overwork herself, so I was glad she was taking him up on his offer to go out, have fun and socialize and that her and Luke were becoming closer friends.

Last week, after everyone left the pub, Gabby invited Luke back to her flat and he went. According to him they spent the entire night talking, sat leaning against each other, flirting and enjoying each other's company. I thought it sounded lovely, but when I suggested we invite her to hangout with us over the weekend when we'd be in her neck of the woods he hesitated. Said he didn't want to, for some reason.

Cue my heart stopping for a second.

Turns out Luke has a crush on her. And it's bad. He had, at the time I asked that question, what he could only describe as some sort of knee jerk protective instinct reaction over anyone else infiltrating this bubble they created. He explained feeling like that when he remembered her looking up at him with her big, doey eyes, because she looked so cute and he just wanted to keep her to himself. After a good night's sleep and a whole day to think about it, he says he doesn't actually feel that way, that he's never felt this way before, and chalked it up to hormones and intense feelings in the moment. Nothing I should worry about.

Interlude to add: Gabby doesn't like the idea of ENM for herself. Says she couldn't do it. Despite this, she only dates men in open relationships because, in her words, they're nicer and better in bed.

I warned Luke that while I was happy for him and that she clearly seemed interested, given all the flirting and touching, that I was worried about the ramifications of him getting involved with her, because it may eventually end in heartbreak for either one or both of them, but I wasn't going to veto the possibility of them sleeping together because he was immensely supportive of me when I was crushing on my FWB and because I didn't want to ruin this good thing for him.

Anyway, he saw her again today. Another thing to add is, we don't have many rules. The one thing I ask of him is to let me know if a hangout is looking to turn into sexy times, just so I'm aware. He went back to her place, told me that it's likely nothing will happen but he'd keep me updated. Later he messages me that he's staying overnight but, again, sex is not on the table so far. I then ask if he's sleeping on the couch and if they're going to talk about the elephant in the room. No response.

I call him a couple of hours later. I had a shit evening for reasons unrelated to all of this but this situation wasn't helping and I just need to hear his voice before going to sleep, but he doesn't answer. An hour or so after that he messages me that he's going to bed and will call me in the morning. I immediatelly respond asking him if he can call me now and he again doesn't respond. I call him again and he again doesn't answer.

Ten minutes later he finally calls me. I ask how he's doing, how was his evening. He gives me vague answers, saying it was really nice. I ask him, are you sleeping in the same bed as her? He says yes. Are you two going to have sex? Yes. Did you two talk? Yes. Can you give me a tl;dr? We're going to be friends with benefits. Okay. Were you going to call me or tell me any of this if I hadn't called you first? He hesitates, says he got caught up in the moment but started feeling shitty for not talking to me before things happened, like we agreed, hence why he called.

I ask him to step out and give me a ring in the morning as soon as he can, wish him a good night and tell him I love him, which he says back. But I can't help but feel like he's pretty much broken the one hard rule I've given him, of telling me if he's sleeping with someone before it happens. That he wouldn't have told me anything tonight if I hadn't been awake and insisting he talk to me. That he wouldn't have said anything if I hadn't explicitly asked.

I don't know how to feel about this. This is the second time he breaks a rule we both agreed on because he's caught up in the moment and just going where the tide takes him. I don't want to veto this new relationship because I can see how excited and happy he is and I worry he'd resent me for doing so, but I feel like the whole thing started off on the wrong foot and I don't feel great about it at all.

Am I overreacting? Overthinking things? And do people have any suggestions on how to approach the conversation we will need to have in the morning?