I believe it is the latter. People who think poly is a sexual orientation seem to think that monogamous people never feel attracted to more than one person. But of course they do. It's just that monogamous people feel the downside of trying to make multiple romantic relationships work are not worth the upsides, and that the benefit of investing more emotional energy into one person leads to a richer experience. It's definitely a choice imo. Which is not a bad thing. Some people choose polyamory and it's right for them. I was poly for a while. But for now I am content to put my energy into my relationship with my husband, he is so unique and interesting anyway that I don't feel I'm missing anything.