Yes, genderized boxes affect both men and women but it is much, much more unacceptable for a man to be “feminine” than it is for a woman to show masculine traits. In fact, masculine traits are considered what is admirable and correct for everyone. Just ask any woman in business or law, or pretty much any profession that isn’t overtly about caregiving.
It’s considered not that big a deal for a woman to be bisexual but it is for a man. Why? Because women let men put their dicks in them. If you do that too, you are acting like a woman and it’s not manly. This is the entire basis of homophobia — 100% of it.
Most of the characters in kids books and TV shows are male because it’s been long believed (although it’s changing a little) that girls will watch/read about boy characters, but boys will not reciprocate.