It's just like males trying to call themselves 'Pegasisters', it's just not right. Yes, you could argue that the word brony originated from /b/ meaning that it's not gender specific; but the 'r' in brony didn't come out of nowhere. /b/ didn't call themselves 'bonys' did they? IMO female viewers of the show are just regular viewers, the difference being they are older than the intended audience.
Btw, not trying to sound sexist or anything, just what I think.