Gender is a social construct, yes, but it always puts a sour taste in my mouth when people use that to "explain" why being transgender is valid.
That's... not why being transgender is valid. If we lived in a genderless society, then surely some people would still have the desire to have traits of another sex.
The real reason being trans is valid is because... well, it is. It's a subjective experience about your identity, what you wanna be, what you feel like you should've been born as, what you are, and a million other things. 
People say that "oh trans people have their brains develop like their identified gender!" but never seem to mention how this can't apply for non-binary, genderfluid, xenogender, etc.
I've said it before, but if you use "the development of the brain" or "gender is a social construct" to validate the existence of trans people, then you're no better than the people who say you need dysphoria to be trans.
I also heard someone say once, in an attempt to "disprove" transrace validity, that their theory as to why transgender people existed was because somewhere in sexual development their brains sometimes switch genders or something. I pointed out how bullshit and unproven this was and they were like "it may not be proven, but it's a plausible theory, and there's no way at all to explain transrace people"
Like, fuck off. If you think there's a need to "explain" transgender people, then you're a bigot. We don't need "explaining".