Me and my boyfriends relationship is dom/sub and unconventional, he also wants to dominate another girl, not sure how to react?

Anonymous
Me and my boyfriend relationship is dom/sub and generally unconventional, we've both discussed before that we'd be ok with the other having 1 night stands etc. For a while I've known that he is really close with a girl who used to live abroad (like super close, before we started dating he told me he had a dom/sub thing going on with her). Since we started dating i was under the impression that hed cooled that off though i know the two still text every day. Since she's in a whole different country i didn't think too much of it though i noticed that whenever he was texting her he'd angle his phone away from me. This year she moved to my country for uni, and I started feeling a bit uneasy when he asked me if i was fine with him meeting up with her yesterday
I mumbled a yeah, but later i snooped on his phone and saw that theyd both texted each other "i love you" frequently. I was devastated and confronted him about it and he seemed genuinely confused. I think he's telling the truth when he says that he thought i was aware that something was going on because hed told me from the start that they did dom/sub stuff, but he also said that whilst he cares about and "loves" her its not in the same way at all and he wouldn't want her for a girlfriend. I asked him what he considered their relationship and what he wanted to do with her etc and he said that he'd never do anything with her without getting consent from me first. I learnt that he'd been planning to visit her over the holidays and it soon became clear that he didn't even consider the possibility that i might not want to consent to him "spanking" and "cuddling" her because id said id be ok with him having 1 night stands. I told him this is different because here its like he's in a relationship with her so i wouldn't want him to do anything beyond a hug. He was really upset by this, and agreed to let her know but he thinks this will ruin their friendship completely and i know he cares about her. I feel im wrong for making him choose?
Me and my boyfriends relationship is dom/sub and unconventional, he also wants to dominate another girl, not sure how to react?
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