He's just having fun. Judging from his YouTube channel, this guy is autistic, and he's a little bit weird, but I can't blame him for that. Kudos to him for being vocal about MLP in public.
I think people like it because of the shock factor. MLP is an innocent cartoon about colorful ponies. When dark elements are added to that, it's even more disturbing than a grimdark story that's not about ponies, at least in my opinion.
If someone was making fun of me for liking MLP, I would just ignore them. I don't need to listen to how MLP is made for girls and how the ponies do nothing but have tea parties and how bronies are a bunch of losers and things like that. It's a waste of time to deal with that.
Really? Most of the haters that I know of are male. Females are usually more accepting of bronies than guys, but not always.
To answer your question, I don't think anything would be different if there were more females in the fandom.
If I were you, I would tell him that FIM is not the same as the older cartoons and that the woman who created it worked on Powerpuff Girls and Foster's.