I don't think any 9-12 year-old would be interested in those stories. Unless one of the consenting adults was a vampire.
Perhaps. But I'd point out that kids - young kids - consume love stories all the time. I don't really buy the idea that they're interested in stories about love the emotion but not love the act.
I was thinking more about stories involving non-loving relationships, sexual abuse, sexual relationships between children, sexual relationships between children and adults or between children and animals. I'm not sure those would always be appropriate.
I'll just point out that there are parents who read their kids Bible stories about all of the above and more from a young age, for better or for worse. I'm also quite sure that there are many children's libraries which stock versions of the Bible.
But putting aside what I personally think is healthy for kids to read or not read... Now, also keep in mind that there are a lot of levels here:
1. Whether some organization should ban books from libraries.
2. What books libraries independently choose to fill their finite shelf space with, or not. Presumably this mostly relates to market demand, cost considerations, etc.
3. What books children show an interest in reading.
4. What books parents want their kids to read.
I think any parent that's worried about that have more than enough adequate tools to stop their children from reading something they don't want to read without banning the book entirely - they can accompany the kid to the library, tell the kid not to go to a certain section, talk to the librarian to make sure their kid doesn't check it out, etc. And I also think that any halfway decent parent is more than capable of preparing their kid for what might accidentally be found at the library, or dealing with it should the kid accidentally find.
I think those that feel the need to escalate it all the way to the level of book banning aren't really doing it for the kids - they find the content so offensive that they don't think
anyone should be able to read it, even kids of parents who approve and kids who can handle it. And that's what I'll always have a problem with.