I believe that is true in some cases. There was once a certain infamous anti who, immediately upon it being pointed out that he focused all of his efforts on catching only those who "prey on" boys, made a half-assed token effort at throwing some girl things up on his web site. Based on his lifelong obsession with sex and boys, it was fairly clear to everyone he was a delusional, self hating BL.

However, I don't believe that is the case all the time. A lot of the online antis, the perverted justice types for example, don't really care one way or the other about the issue. They just like pretending to be detectives and feeling like they are a part of something. They enjoy the feeling of power it gives them when they manage to ruin someone's life by outing them. They love that rush of having ruined someone's life and they can justify it to themselves by pretending the guys deserved it because they were perverts.

Still others probably really do believe the popular image of a pedophile. Honestly if pedophiles were really the kinds of people popular media portrays them to be, I would hate them too.