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if you are talking to a group that contains men and women, you have to use the masculine form of "they": ils (even if there are lots of women and just one man!)
Given the above point, what happens if we address the given group with "elles"? Will the only man in the group take offense? (Because he has somehow been ignored!). How much do the French people care about this?
It is not more offensive than using "she" instead of "he" when talking about a man...
"you have to use the masculine form" : please note you actually use the neutral form "ils", which is spelt the same as "ils" (masculine form).
"French people" means many different people, with different reactions, so I guess there is no simple answer to your last question.
It really depends on the context. But considering my own experience as a long haired man, most of the time when people realized the group contained not only women, then they apologize.
It may be rude for some to be confused for a lady.
It IS mocking, but it is also a common joke.
If you feel comfortable enough with doing that kind of humour with the well-accompanied man, it will generally be OK.
If there is no joke intended, stay neutral and avoid it.
