Okay. I'll admit that Flash Sentry wasn't handled as well as he could and should've been integrated into the film more seamlessly if he had to be there. He also could've been more memorable of a character even if he was a side character there and could've been an honorary member of the HuMane Six too.
But I'm sorry to say that you REALLY go overboard with this.
First. In regards to Twilight having a crush on him… so what? While I personally would've left them as just friends to quell shippers, crushing on a cute boy is hardly something Twilight is incapable of. I'm sure many have had those sorts of moments in real life (not referring to anyone specific).
Now for your claims about sexism. *sighes*
Look, when it comes to the media at large, there’s a large representation of males in fictional characters (of the straight and white variety). Almost every show out there is aimed at male audiences by default. By no means does this one show somehow "oppress" guys.
Oh, don’t get me wrong. We have a good number of well-done female characters. I love The Legend of Korra for this and I'll defend it until my last breath. Yet even those characters and their shows are often overlooked for the guys. Hell, the show wasn’t green lit until Nick was sure a female protagonist could work for male audiences.
In addition, many female characters in the general media are degraded for the sake of a male character far too much. Supernatural, superhero movies and comics, Shonen Anime, the list goes on. Compared to My Little Pony and its "misandry," that's a drop in the bucket. Honestly, girls are lucky to have something like Friendship is Magic to show a variety of female characters. Digibro goes into detail about this:
I guess what I have to say is that in terms of how male characters in My Little Pony are portrayed, it’s hard to call it “problematic” when many female characters in the media aren’t treated as well as they deserve to be. From having their agency removed for the sake of a male character’s narrative to killed off in a very cheap way (Women in Fridges, anyone?), we dudes have it good when it comes to being represented in the media. MLP:FiM reverses that and suddenly male fans aren’t pleased with how their gender is portrayed.
Also doesn't FiM have plenty of male writers and directors on staff?
Lastly, I don't think Flash needed to be anything more than a secondary character. My issue was that he wasn't memorable enough and didn't have much impact as a character overall. That's about it.