Boys don't like girls

That is 90% of it. Boys think the girls have cooties, and therefore so do BLers.[*]

The other 10% is (I remember Remo expressed it better in a post he made) heterosexism in society: since men are expected to like females sexually, the men who don't end up resenting them. For us as GLers the confrontation isn't complete, because even if we don't like women in the post-chrysalis stage, we do like them in the larvae stage. So, of course we resent the jealousy many of them show for the others, like the many who abort or infanticide girls qua girls; but we don't necessarily think women are evil, at least certainly not qua XX. But they don't have that barrage against hating females qua females, as they don't like XX larvae any more than they do XX winged. Summarizing, combine not having an affinity for females at any stage, with the social pressure to go find a female, and you got the recipe for hating all females alike.

[*] I should be fair and add that a lot of it comes from innate differences in the characters and way of acting and playing of boys and girls; but I find even on the pink side that it is not an issue that seems well understood by many, so it is probably not in the BLue side either.