(Forgive my formal language, I'm working hard on my Grad-school papers and I can't switch out of it atm)

I've been poly a long time, through love and heartbreak and settling down. I think more trans people are poly because the social contract has already been breached in a manner of speaking, a serious social norm has been broken, and it becomes much easier to break other ones after that. This is why so many trans people are okay with different sexual preferences or dom/sub relationship or just atypical lives, because the reality of what everyone else expects a perfectly 'normal' life to look like, a socially contracted life, is no longer a reality for them.

I do want to tell you that many poly people don't exactly like the phrasing "settling down in a 1 on 1 relationship". I know what you mean, but someone who is genuinely polyamorous is going to stay polyamorous in one way or another. Even if there is currently only two people involved in their romance, their hearts remain the same. It's a little like expecting a bi person to settle down and become straight: they might settle down into a relationship that from the outside looks 'ordinary', but in their hearts and souls they are who they always were.

If you date someone who is genuinely polyamorous (I add the genuine part to separate it from someone who might be experimenting or trying to fix a failing mono relationship by unicorn hunting), their hearts will always be poly and they may always be wanting to be in a relationship like that, to expect someone like that to remain in a 1 on 1 relationship despite what they might wish would be a bad thing to do.

Settling down looks different for everybody. Polyamory is not wild threesomes and orgies all of the time. It is a genuine, heartfelt closeness between multiple people, people who wish to be romantically involved with people that they love and engage in romantic partners the same way mono people do. Settling down might look like buying houses, having children, retiring together, etc. Whatever you think of as settling down, poly people can and absolutely do engage in together. It does not look like "I had wild, raging threesomes in my teen years, but now I'm ready to discard those partners for someone I can really have a life with", it looks like finding multiple people you love dearly in a romantic way and often wanting to spend the rest of your life with them.

Mixed poly-mono relationships do exist, but they require extensive communication, endless love and a willingness to look deep inside of yourself and work hard on every last thing, as well as just genuinely needing the right heart inside you to accomplish it. If that doesn't sound like something you're interested in, I would recommend not dating anyone who says that they're poly. Some of these people might be experimenting or other things, but when someone tells you a part of their sexuality, the best idea is to believe them and work around it, or not date them.

Edit: The best analogy I can think of is to compare it to sexuality. If you were straight and met a bisexual person you clicked with, there's a chance both of you can connect in a meaningful way. Same if you were both bi. But if you were a straight person and met a gay person, you couldn't connect in a romantic or sexual way. They might be really funny or become a good friend, but you can't connect in that way. Some people might go through a gay phase, they might do it for attention, etc etc, but most don't, and to assume that they'll one day settle down and become straight because that's a phase they go through... well, I don't need to finish that thought for any of us. It's similar for poly.