I'm inclined to think that the primary reason is how sexually repressed the average person is.

I'm thinking of it this way:

The average person hides their kinks, and normalizes the utter segregation of sexuality from life in general in rhetoric. Obviously life isn't separate from sexuality, therefore any display of mainstream sexuality from which there's no hiding, is deemed non-sexual. Hyper-sexualized tv shows, sexualization of young people, cishet couples walking hand in hand or even kissing publicly are ignored to a large degree. Of course there are puritans out there but we're talking about the average person here.

When all this contradicting behavior pushes the cognitive dissonance to unbearable degrees, the average person looks for a target to lash out, to blame for the stress their own sexual repression creates within them.

As a result, the sexually repressed average person can't see, hear or think about any aspect of the lgbtq+ community without their own sexual urges boiling over like an overfilled pot of soup. They think the difference between themselves and the openly out members of the lgbtq+ community is that the average person knows how to keep their kinks, fetishes and sexuality to themselves, while the lgbtq+ community makes a culture out of showing off theirs.

They literally can't understand being lgbtq+ isn't a kink or a fetish, nor is it solely based on sexual expression, because their own sexual expression is normalized into everyday culture for everyone to see 24/7, thus a gay couple walking their kid to school in the morning is no different to them than themselves getting a footjob on the sidewalk next to the school.

At least this is my two cents.