And a lot of people don't. That's always been the case, but because our culture tends to erase those identities' visibility and expression by enforcing heteroconformity, sometimes legally or with violence, it simply hasn't been as apparent.

Also, what 'heteronormative' is changes over time. It's perfectly heteronormative for men to kiss one another on the mouth in some cultures. And ancient cultures didn't even recognize our modern 'homosexual/heterosexual' axis, preferring to consider sexual behavior as normative within class or tribal lines, but considering same sex and different sex attraction as being not worth distinction.