Well first of all welcome to this site, where I assure you nobody will make fun of you for being a Brony.
Second, your parents seem like jerks. They ground you because you like something they don't, and the worst thing is you really can't argue with them. They pay the bills, they give you food and a place to live. I would suggest showing them some female Bronies, or things made by them, to show that it isn't just guys who are into it. Maybe show them this site, and how people are passionate over this awesome show. But make sure you steer away from the pornographic parts of this site. Here's my last piece of help: Take a show that they watch reguarly that you hate, and use your parents argument against them. They'll hate you at first, but they may come around to your side of thinking. I hope this helped, and I wish you luck.