I think you're partially missing the point of the post. Or maybe I am. But I think it's much more about directed, intentionally vindictive behavior and prejudice towards certain groups and subsequent hypocrisy, rather than what you're saying which is focused on sexual preferences. Yes, we all have sexual preferences, but to tout a level of social activism (like body positivity) and then shit on skinny twinks for being skinny is hypocritical and going beyond a sexual preference. At that point it's just nasty, unnecessary behavior.
I also don't buy into the young and unaware of one's surroundings excuse either. Unless you've your blinders on intentionally it's not hard to realize when certain behavior is appropriate. Young people shouldn't be given a hall pass to be a jackass because they've only been on the planet for X amount of years - and this is coming from a young person.