But if there were no gender difference, there couldn’t be gender inequality; one group can’t be widely believed to be superior to the other unless there’s an Other. Hence, #1 is important for #3.
Affirming the gender binary also makes everyone who doesn’t fit into it invisible or problematic. This is, essentially, all of us. Obviously it’s a big problem for people who don’t identify as male or female or for those whose bodies don’t conform to their identity, but it’s a problem for the rest of us, too. Almost every single one of us takes significant steps every day to try to fit into this binary: what we eat, whether and how we exercise, what we wear, what we put on our faces, how we move and talk. All these things are gendered and when we do them in gendered ways we are forcing ourselves to conform to the binary.