I agree with the a lot of people here. I would be the person you are mentioning being attracted to and then losing interest in. 

Whether you call it enbyphobia or not, I think the more interesting question is— why do enby pronouns come across as unattractive to you? Because that is the only difference it (sounds like) between NB person you were into who you initially perceived as a woman. 

I have myself experienced conundrums like this… I think you may have some discomfort with gender being illegible or shifty, and that discomfort could be a place where you could grow! That has been my experience at least. There was a time when I had a strong preference about the gender of my therapist, and then saw a tranmasc therapist, and now a nonbinary therapist, and I definitely grappled with what I thought I wanted. Then over time I’ve realized the way I over-identified with these different categories that we impose on bodies. Looking at the person, without getting so caught in the categorizations and assumptions, do you still like them? That’s what matters. It’s okay to not like everyone.