The non-binary catergory confuses them, or we are frequently conflated with crossdressers - though transsexualism overlaps into crossdressing, crossdressing obv. doesn't necessarily overlap into being transsexual... and then there's the apparent difference between transgender and transsexual which isn't evident to them because frankly the label of the former keeps changing and having things added to it.
I'm not surprised many of them are confused because there are so many things being put under the trans umbrella currently. I'm having difficulty keeping up with this stuff myself. Some of it is voluntary and some of it I class as a medical condition or malaise for which people can get treatment. When I talk to the average non-trans person (including those those are pretty much opposed to our existence) at the moment the general feeling seems to be that those who seek medical treatment have a "mental illness", which I often work hard to correct and explain properly to them. In that sense they think it's not a choice. But then there are those who also think this is a fashion trend resulting from a corruption of traditional attitudes, and again I work hard at explaining to them what being a transsexual is all about and feels like. But I can't explain what a non-binary person is or feels like because I'm not sure. And I can't explain much about genderbending as a pastime or anything like that... except to say that is very different from being a transsexual.
I think they would have an easier time of grasping it if there was a clearer delineation in general discourse between transsexuality and other things under the umbrella.
There is increasing evidence to suggest there are definitive biological causes of transsexuality, some of them would be obvious even to a cis person; but the problem there is that there's still much research to be done to establish a firm link between say, the brain's sexually dimorphic structures and trans identity and other "invisible" things. But the more obvious stuff can easily be explained to them - chromosomal aberration, intersex and the presence of male and female tissues and structures in a person, hormonal imbalances or gland problems, etc.
There's two arguments currently going on in the public discourse though - one is that gender is innate to the brain and the other is that there is no such thing as gender in the brain. One part of our group seems to be working towards being allowed to be accepted as the opposite gender when they need to be, and another part is trying to abolish gender altogether in a sense and make non-binary or no gender or "X" gender a mainstream and legal idea. As I said before, with all this going on, it doesn't surprise me the cis people watching it are bemused. The scientific community is leaning towards the idea gender is innate to the brain, it seems. The LGBT activists seem to lean more toward the idea gender is a social construct.