i'ma put this the best way i can because (for some reason) this is more divisive than some of the more serious issues in our country.
When I say there are two genders, I do refer to gender expression. The third gender that is brought up, or what are commonly referred to as "non-binary genders" is what I consider to be gender non-conforming, which isn't really a gender so much as it is a state of lacking a gender identity that fits. This is what it is to me personally, and based on what I've seen online. People who identify in the non-binary spectrum definitely present themselves in different, unique ways than male and female. However, in my belief, that is more of a gender expression rather than actual biological causes (excluding the possible differences in brain chemistry). I don't mind people expressing themselves as non-binary either. I do believe there are limits, however...
This is where the situation gets a little sticky. I like to call this situation the "Tumblr Mindset", which is, in my definition, when people will make up some unheard thing to feel unique, and then people will jump onto that trend. I know that it's common to blame Tumblr for the many genders being named, but Tumblr has done this type of thing with depression as well. People on Tumblr are also good at raids, which is why it's common to associate many popular franchises such as Star Wars and the MCU with SJWs due to the sudden posting of articles against media for "not having enough gay characters" and such. (That's a topic for another thread) Due to SJWs doing raids, their origins being Tumblr, and the non-binary aspect being attached to the Tumblr Mindset, this allowed for the rising of multiple gender identities which I simply put under "gender non-conforming".
(I know this next part is more than you asked for, but.... i gotta say this)
Of course, if the Tumblr mindset didn't associate multiple genders with transgenderism (a concept that has existed for decades prior), then the transgender community would arguably be in a better place and the concept for gender non-conforming wouldn't have been viewed with as much infamy as it is.
unfortunately, that's not what happened... and this may sound surprising coming from a trans person, but this is where i draw the friggin line...