The bottom line is that MLP is just a TV show. People used to discourage girls from watching "boys" shows like Power Rangers, Transformers, and TMNT, but now it's okay. Many of those shows have been incorporating an increasing number of female leads in response to that.
If it's okay for girls and women to watch children's boys shows, then why wouldn't it be okay for men to watch MLP? While it is geared towards young girls, the show still is well-written, entertaining, and encourages good morals.