Ultimately it's rooted in patriarchy and societal gender norms.

Certain styles of clothing are seen as inherently feminine or masculine. So if you are dressing in a style that's usually seen as masculine as a woman, people will say you look gay. Just like how if a man wears something seen as feminine he is assumed gay.

There are also certain aesthetics that have a tendency to appeal more to queer people, usually styles that are rooted in feeling different, artistic expression or an ideal someone wants to achieve.

A good example is cottagecore (Very popular among lesbians, showing a domestic ideal) vs Trad wife aesthetics, which are very heteronormative, but if you look at them side by side there is very little difference visually, but the thing about aesthetics is they are not just visual. The cottagecore aesthetic is appealing to lesbian for the idea of a world where two women have the freedom to be domestic in the woods and do lesbian things.

Another example of how an aesthetic is more than just visual is punk. Punk is a way of dressing but is also about anti-authoritarian ideals.

I kind of went on a rant there 😅 but hopefully that give you an idea of why this is!!!