Hell, you just have to look at all the Lesbian polar-bears, or the intersexed Salmon, or whole breeding populations of sea-fish spontaneously changing sex, or, or, or to know that it's biological, to know that it's on the steep increase, and to know that, while gender variation has always been a natural thing, the dramatic increase in intersex, homosexuality and transgender over the last 50-odd years is our fault.
Personally, I can't see how all those parabens, xeno-oestrogens and all the other crap the petroleum and plastic industry has polluted our environment can't not be affecting us. (Not to mention those nice hormones and anti-biotics and stuff we use in our food...) Humans keep thinking they're somehow exempt, but we're just like every other organism on this planet. It might be our crap, but it's still killing us, same as all the rest.