At first, yes, I wasn’t capable of anything more. However, I had no awareness of their non-monogamous status. Lesson learned, I will ask better questions in future.
Be aware that asking questions is a good idea. However, you may get lies for answers. More on that below.
The difference for me is that my wants changed and I communicated them. Only then did they share that they were in an open relationship (this was about 2 months in and plenty of unprotected sex).
Wait a minute! You had lots of unprotected sex with a brand-new dating partner? Never mind the casual/committed wonderings. I hope you have already gone and got yourself tested. Even if your dating partner and you shared sex labs results before the first time you had sex, if he lied by omission about his dating status, he might be having unprotected sex with any number of other partners, putting you at risk. Run, don't walk, to your clinic and get tested!
Generally most polyamorous people share sex labs before starting to get sexual with a new partner, and then continue to use condoms for at least a few months, until trust is established and you are fully aware of whom else they are dating and what safer sex practices they are using with them.
They claim my experience with them has only changed due to my awareness of the other person.
That sounds like he is passing the buck onto you, for your choice to end the relationship, when he was at fault for not telling you he had a serious commitment to someone else in the first place. Of course, if you thought he was mono for those two months you were dating him, only to have him tell you he wasn't mono two months and many sex sessions in, you aren't going to trust him and are going to decide to dump his ass.
Hell, even if you were fully polyamorous, you might still dump him for not being upfront about his serious r'ship with someone else. I wouldn't blame you at all.
My struggle is that I feel it was non-consensual, but I don’t know if I’ve expected too much and if my expectations are unreasonable.
It's not a "feeling." It is a fact that you were not given the opportunity to make an informed choice to date this person who is in a
non-ethical open relationship. He is not ENM. He didn't tell you about his other partner(s), so he is non-ethical. He probably does this all the time. Poly men are not seen as good dating choices by many women. Most women are mono, after all. So he chose to lie about his dating status just to get into your pants.
I’m not against open relationships. I just don’t want to be the casual/physical relationship for someone whom I’ve fallen for, especially if they have emotional ties elsewhere.
So you're thinking you could be in an emotional/romantic/sexual relationship with someone else if they were only in casual relationships with others? You're interested in monogamy, being someone else's ONLY love. You're not polyamorous. You don't want to be in love with someone who loves more than one person. That's fine. Polyamory is not for everyone.
This isn't the same thing, but, I have been lied to by men who said they were polyamorous, only to find out a few dates in that they were actually not poly, they were just "playing the field" until they found Ms Right. One of these men told me they were dating a woman who had a kid, and they were sure they didn't want to get serious with someone who had a kid, only to dump me and commit to this woman and her kid! Another man told me he wanted to keep dating me, but the other woman he was dating wanted to be mono, and even though he wanted to remain poly (in his heart), and liked me/was attracted to me, he chose to be with the mono woman (with the intent of being sexually faithful to her.)
I once had a few dates with a younger Indian man. I quizzed the heck out of him in chat and on our first date, because of the culture in his former homeland, about his intent to get married. I asked him if his parents/relatives were not trying to find him a young Indian woman in the US to get engaged/married to. He said, Oh no, there was no pressure from his family in India to marry. He wanted to be with me, he was a swinger, he was into open relationships. After we'd had sex twice, he then told me he was dating a young Indian woman an Indian COWORKER had set him up with. And this woman was seeking an Indian husband! It seemed like he thought of me as some kind of unpaid sex worker, a concubine, who was good to be put to use for his own sexual release, but not worthy of true long-term partnership.
Men do say the darnedest things just to get laid. The omit info, they skirt the edges of the truth, they outright lie. Men admit among themselves, as a badge of honor almost, how they lie just to get laid. Hahaha they are so clever. "Yeah, man, you got laid by that dumb bitch, go you!"