Partner (26M) wants to open relationship after 5 years — I’m (25M) not sure I do

TL;DR: My partner of 5 years wants to open our relationship because he thinks it would be fun and exciting, and says he’d be a bit disappointed if I said no (though he’d get over it). I’m conflicted — the idea sounds interesting, but I already feel anxious and like I’m not “enough,” and I don’t think I’d handle it well emotionally. Not sure where this is coming from, especially since he’s usually not very sexually proactive with me.

I (25M) have been with my partner (26M) for almost 5 years. Overall we have a solid relationship and love each other a lot, but communication has always been a bit of a weak spot for us.

Recently, we had a conversation about opening the relationship. This isn’t the first time it’s come up — in the past it’s usually been more of a hypothetical or something he suggested for my benefit, since I’ve expressed frustration that I’m almost always the one initiating sex.

But this time felt different.

He said he genuinely wants to open the relationship because he thinks it would be fun and exciting — like meeting guys at bars, the gym, or even on campus. He also said he likes the idea of me being with other people, or possibly us doing things together as a couple. He mentioned he’d probably be disappointed if I said I wanted to stay monogamous, although I do think he would ultimately get over it.

The thing is, I feel really conflicted. On one hand, the idea sounds exciting in theory. On the other hand, I know myself — I don’t think I’d handle it well emotionally. I feel like I’d end up overthinking, especially if he was seeing people separately.

I’m also already noticing myself spiraling a bit. It’s only been a few minutes since the conversation ended and I’m hyper-focusing on whether I’m “enough.” He’s told me I am, and that for him this isn’t about something lacking in our relationship — he says he just thinks it would be fun and exciting, like any other activity someone might enjoy. He’s also said he’s able to completely separate sex from the person, which I don’t think I can do in the same way.

What’s also confusing is that he’s not a very sexually proactive person in our relationship. I’ve been the initiator almost the entire time we’ve been together. Even when I visit him (we’re medium distance), there are still times he’s not interested in being intimate. So this sudden enthusiasm about opening things up feels really out of character.

So now I’m left wondering:

Where is this coming from?

Is this about something missing in our relationship?

Am I overthinking this, or is it a red flag that he’d be disappointed if I don’t want this?

I love him and don’t want to lose the relationship, but I also don’t want to agree to something that I think will make me anxious or unhappy.

Has anyone been in a similar situation? How did you navigate it?