I guess because a lot of shows make stereotypical characters like that, and the article writer didn't know much about Friendship is Magic, it seemed like on the surface that Rainbow Dash was supposed to be a stereotypical lesbian character. Because of not being stereotypically feminine and having a rainbow mane.
Feminists are people too and make mistakes and prejudgements. I don't like that people use things like these to be against feminism. Lauren Faust herself is a feminist.
I think the sea serpent Stephen Magnet may supposed to be a gay stereotype. His sexuality isn't stated, but it seems very likely. Actually, not many characters in the show have their attractions mentioned, other than Rarity and Princess Cadence.
To be honest, while stereotypes are bad. It would be nice to have more LGBT characters in shows. It seems like LGBT characters are very missing from children's shows. Because I guess people think it's bad for children to seen LGBT people. Which is really sad and offensive. Even Vivian, one of my favourite Mario characters, was censored for being a transwoman.
... I fear the Hub wouldn't risk having an out LGBT character in the cast. :<