I don't know what you mean about gender is a thing - they believe gender to be purely a social construct.
But gender is largely influenced by our biological sex.
Those that have Y chromosomes are male, those that lack Y chromosomes are female. There are even some activists who deny this reality.
There are those who argue against biological sex existing by bringing up gender dysphoria and transgenderism. Whenever trans-activists are asked to estimate the % of the population that's transgender, they never answer. This is because the % is tiny - the most generous estimates put transgender people at 0.3% of the human population. So, at least 99.7% of the population is born in the right body. The % of trans people is statistically insignificant.
Idiots split their believes into that conservative/liberal shit (all of them, left, right, libtards, alt-right, and whatever made up shit you claim people are. Yes, you read right, I called it all shit because it is) - the political spectrum exists whether you like it or not ...