My boyfriend [24M] expressed that he would like to see me [21] kiss another girl.

My boyfriend mentioned that he has a fantasy of me kissing another girl, which made me really upset. I’m a 21-year-old woman, and my boyfriend is 24. We've been together for over a year, and our relationship has been fantastic in every way, including a healthy sex life and shared kinks. We’re completely in sync and satisfied because we share the same fetishes. We’ve both agreed that we would never consider opening our relationship or having a threesome. That’s just not us, and my friends and I aren’t the type to get drunk and kiss randomly. Last night, during a phone sex call, he encouraged me to share more of my fantasies, so I did. Then he surprised me by saying, “I want to see you kiss another girl.” I went silent and stopped what I was doing. When he noticed, he asked what was wrong, and I requested him to repeat his statement. Here’s how the conversation went after that: Me: Why would you want that? Him: It was just something that popped into my mind when we were talking about fantasies, but if you're not into it, it doesn’t mean anything. Me: So, you want me to cheat on you? Him: No, it's not cheating. Some couples are okay with that and give each other permission. Me: Is that what you want? Why are you thinking about someone else? Him: No, I don’t want that. It was just a fleeting thought, and I don’t actually think about anyone else. It was just about you. Me: I just don’t understand why you would even say that. Him: I told you it was a spontaneous thought, and now that I know you don’t like it, I’ll forget about it. It's not important to me, and I don’t want you to think I truly want that. I’m sorry. While I was getting angrier and speaking with an edge, he remained calm and insisted it was just a momentary thought, not something he genuinely wanted in real life. I confronted him about how strange it was and asked how he could say he loves me while expressing that fantasy. He reassured me that I’m the most important person in his life and that he never thinks about anyone else. He emphasized he wasn’t actually asking me to kiss another girl. I then asked him to consider how he’d feel if I said my fantasy was to kiss another man in front of him. He understood my point and kept apologizing, reiterating that it was a passing thought. I accepted his apology. He then shifted the conversation to lighter topics, and we ended up joking around for another hour before hanging up on good terms. Was I overreacting? I just never expected him to say something like that. I’ve given him no indication that I would want to kiss another girl, so I thought he would know that I'd hate the idea. I keep wondering what would have happened if I had gone along with it and expressed a desire to kiss another girl.