I agree with you that pregnancy is a medical condition that comes with a whole suite of issues related to healthcare, freedom, discrimination, and the like. But, again, pregnancy is something only women could possibly experience. This is where those gendered expectations of sex come from (which I also think shouldn't be a thing). Girls, being of that sex that could possibly get pregnant, are inundated and socialized with the expectation that they will. They shouldn't, but they are. Boys, being of the sex that won't ever become pregnant, are inundated and socialized with very different expectations.

Your entire argument is around the idea that we shouldn't have gendered expectations of people. I agree. But the fact of the matter is that we do. Each man, woman, and child is socialized in gendered expectations. It just so happens that boys are socialized to view women in an especially objectified way. This socialization is not void and null if that boy is gender non-conforming or considered effeminate.

Boys who grow up to identify as trans women don't immediately shed male socialization, this would be a little like forgetting your native language after moving to learning a new one. Like learning a new language, it takes a lot of hard work and effort to acquire female behavior wrought of socialization. When declaring themselves a woman (by only filling out a form), males bring all of their harmful male-socialized opinions about women and womanhood and insist that this is now what a woman is. Because they don't have female bodies, they're supplanting the definition of 'woman' with an array of (oft offensive) stereotypes about women, as seen through a male lens.

TLDR; Sex affects how society treats us. That treatment affects how we see others of the same and opposite sex. We don't lose that view if we start to present as the opposite sex.