It's really easy to argue that sexual orientation is sex based. The argument that it's based on gender expression, will crumble easily once you remove the factors of gendered expression and fashion.
Ask yourself.
If a group of people, divided by their sex, were in a room together. You had the females lined up backs against the left wall, and had the males lined up backs against the right wall. They are a diverse group of different races, ages (from 20 and onward), sizes and facial features. All you can tell from them are what they look like. You do not know their names, "preferred pronouns", behaviour, or what they like to dress in. None of them are wearing makeup. They are all wearing the same outfit. For simplicity's sake let's say a gray long sleeved shirt, and black sweatpants, all unisex clothing, and none of them have shaved their body hair. They have the same haircuts.
Which group would a lesbian be inclined to? If you asked a lesbian to pick a potential partner from one of these groups, which group would she pick from? Now which group would a gay man pick from? What about heterosexual women, heterosexual men? Would a lesbian be okay if you made her pick from the group of people standing against the right wall? How would a straight woman feel if you limited her options to pick from the left wall?
Without gendered expression what left would there be to determine your sexual orientation now? Can you finally admit that sex is a crucial part of sexual orientation now? Or are you just going to keep lying to yourself to feel better.
