In other words: there are plenty of reasons to not stock a book that are not partisan or doctrinal. We don't expect public schools to pay for expensive medieval manuscripts, for example, or to stock books in languages that aren't represented in their district.
I don't see how pornographic bans wouldn't qualify as "doctrinal" though they are not particularly partisan.
So if you're attempting to ban it because it's obscene, that's okay. But if you're trying to call it pornographic because the lead characters are in a romantic relationship and happen to be of the same sex... well, that's not obscene, and your attempt to call it pornographic is doctrinal disapproval.
And to be clear: As a rule, I am in favor of librarians making this decision without interference. Librarians have repeatedly put their livelyhoods at stake in order to protect individuals access to information, and protect the privacy of those same individuals.