@hihellohihello
It's for the same reason that I don't get annoyed by suggestions that rudeness, indifference, being aloof, lacking compassion, or having absolutely no interest in the lives of other people can be signs of a ND person. Nobody seriously suggests that ALL ND people are the same, but can these things be present in ND people? Yes, absolutely, I'm one of them.
It's good that you don't get annoyed. However, I think it's the inference that these traits are definitive of neurological diversity that is offensive to people. Of course, it is correct that ND people can have these traits as can NT people.
Well that, I think, is the reason I don't get annoyed. I can't recall a single instance where it was inferred that these traits are definitive, rather more that it was a proposition asking if the OP had ever considered that they might be indicative of the fact the subject might be an as yet undiagnosed ND person.
If someone was to say something as definitive as "well, obviously because he doesn't tell you he loves you and shows no interest in your sister's children he MUST be autistic" then yes, I'd laugh that out of town as being completely ridiculous, no doubt be a bit annoyed by it, and if I was in the mood probably challenge it, but most of the time these things are a simple suggestion that perhaps there might be something in play that the OP hasn't considered, nothing more.
I think it's 99.9% of the time a genuine attempt to perhaps be helpful and contribute something that stems from a good place. After all, just how many ND people with absolutely no diagnosis do we think there are as opposed to the numbers of confirmed ND people? It's a hugely under-recognised phenomenon, so I don't think it's in any way strange that people often posit "have you considered?" and such.
I agree that some of the more trivial reasons for suggesting an undiagnosed ND condition are ridiculous, and there does seem to be a trend for internet diagnosing autism in strangers based on nothing but the flimsiest and most scant of information. The things I'm talking about are mentioned further up the thread, not liking one specific foodstuff, not wearing socks, and so on, but for more significant personality traits that absolutely can be present in ND people, no, I don't find it offensive in the slightest if someone asks if it's ever been considered that the individual might be ND.
To be honest, I find the usual slew of mumsnetters chipping in with 'no, he's just an arsehole' far more offensive. People do not act like arseholes, harm others, or go out of their way to offend just on a whim. Human beings simply aren't like that. There's invariably an underlying cause or reason for those behaviours, so to write it off as something that just naturally occurs for no reason whatsoever is puerile and contributes nothing whatsoever to the discussion. It's the same as the sort of 'just because' answers that you get from 4 year olds.