How do I navigate the marriage talk with my partner?

Backstory. Sorry but this is long, and i'm definitely going to get a therapist but it's difficult honestly. A therapist is one person with a limited, clinical perspective but hearing from people in similar situations from around the world is so much more enlightening so thank you in advance if you make it through.

I've (30 M) been dating my boyfriend (34 M) for 4 years now. First, real relationship for both of us and I'd say it's been a really good but honest relationship. We fell in this relationship with hard feelings for each other after years and years of not connecting with anyone else. We can't imagine our lives without each other. We're both better off with each other: he influences me in ways that make me better and I to him. He tells me he loves me everyday and calls me first whenever he has free time. We're best friends.

It's not perfect obviously. I have to constantly pester him to take things more seriously. Although he is older than me, he acts much more childish when it comes to thinking about our disagreements, his own feelings, his own insecurities, and the future of our relationship.

We started our relationship with the expectation of being in an open but committed relationship. We have fun with other guys often. I'm naturally a possessive person but I also have a kinky side that likes to watch my partner doing sexual things with someone who is not me. I don't like it when my partner does romantic things with anyone else, however. He likes to be flirted with to appease his ego and may edge too much on the romance part to get that sort of attention. That part has caused some arguments throughout but we're continously working through that.

My predicament is... I think I want to marry him. We behave as a married couple. I see all of these other couples and I'm happy for them. I see their partnership and love for one another and I respect it because I can imagine what it took for them to be at that point. We put in the work, the love is there, so why doesn't my partner see marriage as an option.

About him. He's a physician so he's had to do the whole medical school, residency, fellowship etc thing. I met him while he was in fellowship and he finished that while we were dating. From what I gather about that profession is that you don't get a lot of time for yourself and most importantly to work on yourself. That's kind of evident to me in how he thinks. He pushes anything too mentally demanding to the back of his mind and eventually it comes out as an outburst or passive aggressive 'joke'. His work is a major burden on the relationship but I've resigned to making it work around it. He's not out to his family who live across the world and speak a different language than me. (He's been here so long that he's nearly forgotten their language). But he's out to every other important person in his life and has no plans to ever go back to his family.

We (him and I) also suspect that he has undiagnosed BPD and body dysmorphia. All things that we work through and I'm not too incapable to work with him on. I can accept him for who he is because I'm sure it's something that we can continually work through (been working on getting him to a therapist).

But, my question is, why is it I can't get him to open up to atleast consider marriage? His response whenever I bring up that maybe we should just get married at some point since we're already pretty settled with each other is:

"Marriage makes problems in people's relationships" and points to other people's marriages for justification.

Or that a divorce would be messy and a headache. I have insisted that we sign a prenup and clearly line up easy terms for a divorce and allocation of our belongings. (All that we really own together is a car under a predesignated joint ownership agreement).

I mostly only bring this up because financially it's the best decision as we would save 18K on taxes a year from filing jointly. And, I think after 4 years together, it's a sensible topic to discuss. And also, part of me does want it with him. Maybe to say it, even just quietly to myself, that he's my husband. I know he feels the same way, but what's it going to take to show him that it can be that way.

What are your own experiences with talking to your partner about something serious and important like this that they might not take seriously or see as important?