1) they think girls will then give them attention.
2) they'r prbbly just tryin to make it as an excuse to hate him. ex: girls: jb is sooo hottt!!! boys: ewwww, gross, we hate him. girls: why?you guys are just jealous. boys: we'r not jealous! he's just gayyyyy
3)they hate him and they wanna influence girls.
yup! those are my answers! ;p xox
PS. my best friend's a boy and he HATES justin bieber; which is not weird.