I may be biased, but I think polyamory and demisexuality go really well together. To review, a demisexual person is someone who only experiences sexual attraction after getting to know someone very well first. Within these close relationships, a demisexual person might experience strong or frequent sexual attraction, or might experience weak or occasional attraction. Personally, I don’t experience sexual attraction with everyone I become close with, or every romantic partner, but do experience it with some people (there is no clear pattern).
Given this, I find polyamory to be super, super handy. For there to be any chance at sexual attraction, I kind of have to spend some time pursuing someone, get to know them in a deep intimate way, and even then I might end up in a great relationship where I don’t experience sexual attraction. This might frustrate the hell out of a monogamous partner, if being found sexually attractive is an important part of a relationship for that person. But being poly, my partners can easily seek that attraction elsewhere if it doesn’t happen for me. It also means that the variable and occasional nature of my desire isn’t a roadblock. I have and have had partners who really like frequent sex and that’s not a problem when they can simply get it elsewhere.
This isn’t to say that two people can’t be in a monogamous relationship when only one is sexually attracted to the other. Of course there are creative ways to work around this, and some allosexual folks wouldn’t be bothered by one-sided attraction, especially if it doesn’t affect sexual activity. But in my experience, polyamory takes a lot of these questions out of the picture.