Atypical, non-standard, or unconventional are more neutral in tone, so given your desire for a non-subjective word I'd recommend these instead.
I mean why do people even post something like that? It takes 2 seconds to look up the definition of abnormal. It's it really not knowing, it's is it (what I believe) trying to sneak in their moral judgements behind a veneer of supposed "neutrality"?
> Abnormal - deviating from what is normal or usual, typically in a way that is undesirable or worrying.
Yes, that's precisely what it is. Moral judgements based on outdated ("conservative", especially clerical) understandings of the world, wrapped in some delusional sense of "objectivity". Only the scientifically and philosophically illiterate fall for it. In German, we call it Bauernfängerei (swizzling, duping; lit. "pawn catching").
If the topic is about whether homosexuality is non-normative and heterosexuality is normative (with the actual, common meanings of the words), we can have a philosophical discussion on that.
Of course, it's an unprovable conjecture - but it sure would explain a few things.
You fall into the same trap ("non-standard", "atypical"); you just stepped on the euphemism treadmill.