It’s not complicated. If you’re straight, secondary sex characteristics differentiate the opposite sex for you. This is also why men like wide asses–widening of the hips for women is a secondary sex trait.

Women also exhibit this attraction. Conventionally attractive men have square jaws due to more muscle produced from testosterone. They also tend to have facial hair and more muscle definition, all of which are secondary sex traits from testosterone production. Increased height is also an indicator.

I’m not a scientist. But it seems we’re coded to find sexually mature people attractive and those signifiers tend to be secondary sex traits.