This old chestnut. Its not the first time this has happened with manga, since its been gracing library shelves. But like all those other times, I have to back up the libraries on this one.
Parents, its your responsibilty to vet what media your kids consume, tough job but only you can do it. Having an objection to yaoi (pronounced 'ya-oi') manga being in a library isn't going to fly, when 50 Shades of Gray is also available, is lapped up by adult females (the same audience for yaoi) and given the red carpet treatment, because its a text novel. This yaoi book has a parental advisory label on it, so its doing its part that what's inside, may not be suitable for a younger audience.
Manga has done some great things for the comic medium, by pushing the bounderies, in a similar to many indie/creators owned comics. The regular people on the street still unfortunately see it as stuff for kids, and how wrong they are.
Sounds to me, that this is more of a 'gay's are evil (not true), so why are there comics about them, in our good library' witch hunt. The moment they start sensoring and burning books, I'm afraid they'll have to stoke the fires with 50 Shades trilogy, too. I'm amazed that such hypocrisy still exists that whatever is produced in text, gets a pass, but if its drawn, then its somehow a problem, and more corruptable.
As for such a yaoi book, take a moment to talk to your child about it, and why they shouldn't be reading it.