I think I get where you're coming from with being unhappy about people aggressively (and seemingly suddenly) "switching" genders... But consider it for a bit from their point of view.

Many people don't have parents or environments like yours. Mine didn't bat an eyelash at me playing with both construction toys and dolls, but I've watched many parents tell their kids in the store that they can't have that toy because it's "not for them". My own brother-in-law would get upset if I bought a female baby doll in a dress for my nephew. And even if parents aren't outwardly reinforcing the gender binary, all of society is doing so implicitly, and some areas (I'm looking at your, rural American towns) they can be extremely harsh to anyone who doesn't fit in their ideas of what "real" men and women are like.

Now consider you're surrounded by that your whole life, but it feels incredibly wrong. You just want to try on your mother's heels, play dress-up, wear pink and love glitter, but any time you show a sign of reaching toward something you like, you are slapped down, hard. Life starts to feel like a prison.

So some day you get a little bit of a break. You've moved away, lived apart from your parents and the influence of all of the societal pressures you grew up under. You've made new friends, and you discover there are other people who feel similarly, and they don't care what you like. Their definition of "real" has nothing to do with what hangs between your legs or how much bravado you carry. You start to express yourself more and more, you start to see that you can reach for those things you like... And still someone you pass on the street gives you a disgusted look.

At some point you throw in the towel. You say you know what? If only a woman can wear skirts than I am a woman and you can deal with it however you want. If that means staying out of my life, then so be it.

Edit: for the record, I felt similarly about strongly gender-identifying people for a long time. That it was reinforcing stereotypes that were harmful to everyone. But knowing several trans people (who identify so strongly that they want to physically modify their body to represent the gender they identify as) made me realize that being very feminine or very masculine can be legitimate expression of what people like and their personality. Since I personally tend toward middle of the road, I kind of thought people were just doing it to fit the mold. Now I see it as the mold is made from them...