Beyond this though, on your second point I don't necessarily think people have the wrong impression of shoujo. With a few exceptions, shoujo definitely has a "feel" that is difficult to describe. You do a pretty good job of explaining it yourself. Which is confusing that you wouldn't draw the connection that the things shoujo does do is just simply things many people don't like. Not everyone wants a stronger focus on romance or even just relationships, a more "real-life" as you put it feel, or anything of that sort. Maybe I'm generous to assume people at least have a somewhat accurate idea of shoujo when they say they dislike it, but I do believe that's probably the case.
Lastly, I think the "societal constraints" thing is so overstated, especially today. It's even more ridiculous when talking about anime, something that you can watch alone without judgement from others, then talk about online anonymously if you actually cared. Yet in reality, I think there are a sizable number of people that really go against this narrative entirely. A lot of guys like Idol shows, myself included, I even have enjoyed a lot the cutesy things I've watched. Weirdly I think Shugo Chara was one of the first things I watched in that aspect, I really liked it personally. So maybe I'm biased from my own opinion but I don't get this impression from people at large.
The only thing I would add not directly replying to something you stated, is I don't actually think all shoujo is something that doesn't have broad appeal. At least in western anime fandom, things like Natsume, Ouran Highschool Host Club, Fruits Basket, ect seem to have pretty broad appeal. I can see why they're labeled shoujo, and that probably have larger skews of female audiences than a normal series because of that. However, that hasn't stopped them from crossing whatever divide exists.
Anyway I personally really love a lot of shoujo even if it's not a favorite genre of mine. So again, maybe this is all bias to some extent.I think you're reading way too far into this to get any good amount of perspective on it. On your first point, people tend to mock teen girl's interest because they have often been the most loud and obnoxious groups about their interest. It doesn't help that there is some truth to the pattern what they like tends to be a bit trashy. Granted I say this as a fan of ecchi, even the not so great ones, so I'm not throwing stones over here. Anyway, when it's not the case that they're actually onto something good, those things will generally reach acceptance such as with the Beatles as you mention. Trust me, I know it's annoying having your interest constantly attacked, but you're not a victim of prejudice just because people make fun of you or the things you like.
I see that you disagree with my opinions quite alot XD I just want to open my thought process a bit, so maybe you'll see where I'm coming from?
Regarding your first point in all honesty your first point just reaffirms my view, why exactly do you think teenage girls are louder than teenage boys? What exactly makes you perceive their interests as trashy (even if you don't mean it in a bad way) ,compared to the taste of teenage boys that are not trashed? and yes some of the things will reach an acceptable status but in case of for example the Beatles only after the approval of others and the teen girls are usually forgotten as actually making the thing popular or are not acknowledged.
Yes I acknowledge that not everyone likes romance and that is a very valid reason not to like shoujo, I actually personally do not like romance, however what I was trying to say was that it is frustrating that people only seem to equate shoujo to that when there is so much variety and other things that can be found, obviously not everyone is going to like those things, but I wanted to higlight that its important to actually look a little deeper than the glossy surface. As, I feel that people don't give shoujo a chanse beyond viewing one thing and deciding it isnt for them.
On the final point I do disagree with you respectfully, in my opinion the society we are born in shapes us from childhood , how many times has a boy been told not to play with barbies? Or not to like pink? Or a girl not taught how to change tiers? Or not to play with 'boy toys.' I believe these are things we internalize from our environment and therefore these are things that often affect our behaviour in our subconcious.
Hope this cleared my though process a bit :) (And yes I'm biased too)