It's not a "slippery slope," and libraries already are censors.
They don't carry books promoting pedophilia or bestiality, do they?
Why is one perversion preferable to another?
With children's books, there used to be reputable publishers who didn't choose topics that are inappropriate for the age group.
There's another problem that's been around for decades. That means it's affected today's parents. That's the desire a lot of parents to succumb to, wanting advanced children. When these parents choose materials for their youngsters, they should choose materials where the subject matter is age appropriate, but with a higher difficulty level of reading. Instead they have their children reading material above their maturity level, thus beyond the developmental stage they should be experiencing. The other outcome is gaps in the kinds of knowledge and values a child should be experiencing at younger ages.
The best way to avoid that is the classics and children's books at least twenty years old. The difficulty level will be built into the classics, those books a few decades old will be age-appropriate for themes.