I (22M) dont want my gf(37F) playing sexually with her best friend (35M). Am I being controlling and possessive? AITAH?

Me and my girlfriend have been together for a year and 8 months now and I met her at a kink bar. We really hit it off and we are totally head over heels in love with eachother. We've never had a major argument (apart from the one I'm about to explain) and always want to spend all our time together.

We are in a semi open relationship, only having sex with other people if the other is present and we are both into it. We often play with friends but sometimes go to kink events too. We have a very healthy sex life together and will only play with other people once a month or so.

However, she has a best friend who I'll call Kevin, and her and Kevin are super close. As in, send eachother "love you goodnight 😘😘" messages close. I know that he does have feelings for her because she told me so. He is also very tactile and coddling when they are together, which I think is nice, but in moderation. When they're together they are glued to one another and he is constantly touching her. Granted Kevin is this way with all his friends, but there's something different about how he is with her. It's bit too loving, a bit too sensual.

I don't think he's a bad bloke, he's nice enough and we get along alright, he's just very clearly in love with my girlfriend and shows it too.

I've spoken to her before about how this makes me uncomfortable and pushed aside. Weve spoken before about my insecurity issues when it comes to sex and relationships, so I was quite surprised when she turned around and said I was being "possessive and controlling" and that "this is major red flag behaviour" I tried to explain that I was only telling her how I felt, I wasn't trying to restrict or control her in anyway. But she accused me of being "jealous" and "it's really ugly". In the end I conceded as I couldn't bear the thought of losing her. But since then, this has been something that's weighed on my mind.

Sometimes we will be at a kink event with Kevin and he will play with my girlfriend and I feel like I can't say anything because Ill come across as a controlling dick. I only take issue with it because there is obviously a deeper connection there, wether it's one sided or two. If they didn't have that strong emotional bond I wouldn't care. But I'm just hurt, and it hurts to see them like that.

I feel trapped, like I either have to submit and accept this is the way it's going to be or tell her point blank I'm not comfortable with it and I'm 90% we would be over. I love her so much, she's the best thing that has ever happened to me.

She says that I don't need to worry about him, that she's turned him down tons of times and he knows nothing will ever happen, it just doesn't sit right with me.

Am I being a controlling possessive dick or am I being reasonable? Someone please help, I feel like I'm going mad.

*Edit - just wanted to make it clear we dont have any other partners outside our relationship, we are completely monogamous apart from if we are both present and into it.

** I will also say she supports me in everything I do and has been a real anchor for me the last year and a half. Im not sure what I would have done without her