It's difficult. I do have Pony Shirts. My parents have seen them. They were like "that's nothing for a grown-up", but they actually say that about any cartoon so that doesn't really mean anything.
I'm pretty sure my parents have no idea about bronies or the show. And even though I don't think that they would think any less of me (aside from the point, that they look down on cartoons and think that animated stuff is only for kids). I don't really have a reason to tell them. Just like they mostly have no idea what games I play, even though they can see it on my screen.
My friends know, that's important to me. It would only cause me headaches if I told my parents, so I'll just keep up the status quo there on that front and let them keep wondering, why I wear such strange shirts, until they decide to ask me.