I like this anecdote because it contains a non-sexist but inappropriate statement. It also shows that there are at least two possible mindsets:

* I talk to my coworkers like they are my mother

and your colleague's

* I talk to my coworkers like they are my friends

The fact that the statement was non-sexist hints at the problem being the second mindset; as for me, I make sure not to say sexist (and racist and so on) things to anyone but I am okay with talking about sex and other personal-jovial matters with my actual friends.

The "mother" analogy still has some flaws, though -- for instance, you can talk to your mother about financial, health-related and personal troubles, whereas your coworkers will feel uneasy with such information. Another thing is politics -- I try to politely argue against my mother's political views (say anti-immigration), but I think doing so in the workplace is a very bad idea.