There are two main reasons
1. Gender Stereotypes, society claims that only young children should watch My Little Pony and purchase the merchandise and there has been schemes to counter the stereotypes like "Let Toys Be Toys"
2. People who are overly obsessed with MLP, those people are more obsessed with the show than an average member on these Forums. They would very often try to force people to join the fandom and make their pony OC marry a canon character. One example comes from the Google+ user Legitman335, he would usually draw pictures with Sunset Shimmer and really try to date the Sunset Shimmer roleplayers over there. His roleplaying got so severe that he got banned from two MLP Communities there just for roleplaying multiple times, both of which I am a member of.