Anybody would get defensive while being generalised about in a negative way. Men are simply pointing out that the generalisations people make are not correct.

If I say "Women prefer to be at the home rather than at work". You would probably tell me that every woman is different and we cannot generalise about them.

But if somebody says "Men do X" or "Men are X", the generalistion is just accepted.

Some men commit horrific acts of sexual assault. But it's NOT ALL MEN. And I, as a man, am not responsible for the actions of other men.