My boyfriend suggested an open relationship but is upset that I'm dating other people, why?

I fell in love with a guy who I believe is my soulmate. We connect on every level (mentally & physically) and I have never felt so strongly about a person in my entire life. I feel like I have known him in a previous lifetime.

We are in a long distance relationship and certain circumstances and obstacles are getting in the way of us staying together. I am so heartbroken but we're trying to take it one day at a time & not stress about the things we cannot change. After months & months of trying to resolve all the issues that are getting in our way, we both agreed that we should have an open relationship and see other people. The idea is killing my inside because just imagining him with another girl is making me go crazy. I am so sad but I know it's the best thing for us right now.

I asked him if he's truly okay with our new arrangement & his response was "Of course I'm not okay with it & I would be upset if you dated other guys. But we can't get so invested in each other if we have no idea how our relationship will turn out. I don't want to tie you down and not have it work out in the end."

A few weeks later I took pictures with a guy I went on a date with and captioned it "Having some fun!" We went out for drinks and it was a good time but he just wasn't my type. I was honestly just getting my mind off my boyfriend and trying to distract myself. My heart just wasn't in it. He didn't even compare to the connection that I felt with my boyfriend and I just did it so I could mentally move on.

My boyfriend was like "So you're officially dating him?" I was like "No, I just went on a few dates with him. Why are you so upset by this?" He was like "I know you're not mine right now but I'm just upset. I don't know why exactly." Why would he react this way when he's literally the one who initiated the whole "open relationship" thing and I was the one who was so against it? Why does he want me and yet encourage me to move on?
My boyfriend suggested an open relationship but is upset that I'm dating other people, why?
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