Nonsense. Women are generally shorter than men. Women generally have a higher percentage of body fat that men. Women are generally more susceptible to breast cancer than men. All generalisations, and all true. I doubt you think any of them are stupid. So why is recognising physical differences acceptable, while recognising psychological differences is not?

I'm not sure how true that really is. Sure, it's easier to observe someone's height than it is to observe their tendency to go off-topic (incidentally, I'm not convinced that is a particularly female tendency), but the difficult part with getting accurate data on either characteristic is getting a large enough sample. People often base generalisations on anecdotal evidence, which is generally not representative. I would imagine generalisations about easily observed characteristics are more likely to based on anecdotal evidence than ones about more difficult to observe characteristics, which are likely to be based on formal studies, since that's the only way of establishing it (of course, this generalisation is, itself, a counter-example...).