Does that mean you're attracted to them being feminine? Or are you attracted because they're men?

yes

More seriously, in my experience androgynous/GNC features are widely considered attractive in bisexual circles because they combine attractive features of multiple genders, which is at least twice as many attractive things.

However someone discovering themselves as "not straight" might not have enough understanding of the relationship between gender and attraction to answer your question.
In my case as I hit puberty I realized I was attracted to girls, so I thought "ok I'm straight". Years and many ignored signs later I realized I found feminine men hot, so I thought "I'm straight so it's because they're feminine". Then like a year or two later I caught myself thinking a definitely-not-effeminate actor was hot (not for the first time) and the other shoe dropped. Yeah, I'm a dumbass, but point is that it's much easier to put a label on oneself and go from there, than to deconstruct something as complex as gender and attraction.

Thank you for coming to my ted talk.

Thanks for clarifying that you were talking about trans women.