The only fanfic I've ever read(well, attempted to read, really) was Naruto Veangance Revelaitons, and that was more because it's hilariously bad in a way that makes you just go "My god, this guy is either the best troll ever or so spectacularly stupid it doesn't matter."
And yeah, lemonfics exist only so authors can have their Mary Sue self-inserts fuck all their favourite characters.