Mean people, insecure people, and bullies will always try to find a way to criticize others. Throughout the ages, violence and cruelty have been lauded by such people s a sign of strength, while kindness, innocence, and love have been regarded as weak. (Most likely such people who criticize these things have an underlying jealousy, as their minds are filled with anger and sadness, and they continue to feed those though their pursuits.)
It's not just "ponies are for girls". It is the mentality of humanity as a whole. Ponies are sweet and kind, and encourage such behavior. Sadly, there will always be people who, for some sad, inexplicable reason, seem to hate the sweet and lovable, and perhaps even secretly fear it, for it reminds them of how unkind they are and how bleak the world they have made for themselves is.
But on the bright side, bronies seem to really be spreading. Apparently a lot of people are getting tired of the bleak, the cruel, and the violent, and are just unafraid to admit that they want a little happiness, sweetness, and love in their lives.