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 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 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.