Can I even be seen as monogamous to other people?

This post may be long, bear with me - I feel like my past is complicated.

For reasons involving trauma and body issues, I (39M) was what you'd call an extremely late bloomer. I was a sex-repulsed asexual until my late 20s. I always felt so strongly that friendships could be just as close and important as romantic relationships.

I met a friend in my early 20s who I became roommates with, and he and I grew closer slowly and naturally over time. He has noticeable autism, which may be linked to why he is also a sex-repulsed asexual. 15 years later, and we are still living together, sharing the cost of the house I bought - we currently share a bed (without any sexual inclination whatsoever), we do hug, we do not make out, we don't have sex in any form. We have mutual friends and we have our own separate friends that we spend time with on our own. But we love each other, we're best bros, and we're not afraid to say that.

As I worked on unpacking my past, learning to love myself, to trust, and to be vulnerable again, I discovered my sexual (and more traditionally romantic) side for the first time. A deep desire to have a connection of that level arose when I began to fall for a long-time friend. I sat down with my best bro to talk about it, because in many ways, we did operate like a couple. He was fine with me pursuing a possible romantic relationship. I did not do so with the woman I'd fallen for at the time - I took my time - but someone else came along, who was already partnered but described herself as polyamorous, and suggested that I was probably poly too, since I did love my best bro and also wanted another relationship.

It did not work. She would throw her "nesting partner" aside to obsess over me, and then when she felt too close, she tossed me aside to chase a new person. One of us was always playing the third wheel. Fast forward to more recently, I dated a polyamorous woman for 3 years. Loved her deeply, wanted to spend my life with her. After a couple years of intense devotion to each other and what I thought was love, she was off chasing a new high too. If I even questioned things, I was told to back off, that she was poly, and I was encouraged to find other partners to make up for the abandonment, until she eventually dumped me for another man who wanted her to be monogamous with him.

I know in retrospect, these were also unhealthy relationships in which I was struggling to establish boundaries, something I'm learning to do right now. Thing is, I can count on my fingers how many people I've ever felt attraction for. When I'm in a romantic relationship, I feel zero drive to pursue anyone else or even look at them. Attraction doesn't come easy for me, once I love someone, my love for them does not fade. I feel naturally inclined to one person, and am fully satisfied with them. I'm not opposed to experimenting but the poly situations just left me feeling like I was broken and incapable of feeling the way they feel.

It's left me so confused. Friends and partners labeled me as poly because I have a partnership in my life that isn't considered normal for a friendship, but it's also not normal for a romantic relationship either. I would like a mutually exclusive romance in the future, but have been told by monogamous friends that they'd never date someone who had a live-in best friend like that. I spent copious amounts of time one-on-one with my girlfriend, shared the same bed with her, etc, my bro doesn't insert himself into ANY of that and in fact tends to shy away from most people in general. I took my ex on romantic getaways for weeks, we talked about going cross-country for a few months just the two of us, working remotely, so it wasn't like my broship was interfering with my romantic life, at least not from my perspective? I don't think I'm poly and I'm not sure I can even be happy dating another poly person, even if they were treating me more like a person, but I feel like monogamous people are not okay with my deep sense of friendship either. I can't just throw away a 15 year friendship, force him to move out of the house and live by himself. We love each other, it's just a different kind of love - but I also yearn so deeply for a romantic love too.

I'm not even sure what advice I'm looking for here, maybe reassurance that some monogamous people would be open to understanding and considering a relationship with me - or maybe the hard truth from folks that no, I can't expect monogamy in my situation. Just feeling like I have to choose between a possible future romantic partner and my bro.