My long-term fwb asked for a relationship at the worst possible time

I've known my fwb since senior year of high school, and we're both in our mid thirties now. We started casually hooking up in college, whenever we were both single and geographically convenient.

We had drinks, recently. After complaining about work, as we usually do, we ended up talking about ENM and poly relationships. For context, he's had some failed ones, and I am currently in a LTR that started off as open and recently closed up. He then got this look and asked, really quietly, "Would you have a relationship with me?"

I took the cowards way out and skirted the question, for a bunch of reasons. Topping the list, he asked this knowing I am unavailable - not just in a LTR, but making long-term plans with my partner. Second, even though he repeated it and told me he was serious, I don't believe he is. We've known each other for literally half our lives, so if we were going to be an item, it kind of feels to me like it should've happened already - it's not like we lacked for opportunities!

It seems more than a little odd that this comes up only when I'm no longer available - when, if all goes well, I won't ever be available anymore. What's with that? Is this sabotage? Jealousy because he's single but I'm not? What's the end goal here, anyway? Does he genuinely think I would leave my partner just because he asked? Because that's downright offensive. Or am I supposed to believe that, after fifteen years, now he suddenly thinks we're endgame?

It made me angry and sad and uncomfortable, and if I let all of that out, it was going to get ugly. So I avoided answering and went home.

I have been thinking about what he asked, and just can't wrap my head around it. What was he even thinking? This isn't a movie or a tv drama. There's no "It's you, it's always been you" moment coming up. That's just now how life works.

It does make me a little sad that I'm losing my buddy. I can stay friends with someone I used to hook up with, but not with someone who'd think of interfering in my relationship this way. I already chose my person.