It's one of my pet peeves dealing with people who get offended on someone else's behalf, but - having said that - I also encourage anyone to speak up when they feel that an injustice has been committed.
I don't think somebody can ever really take offence on somebody else's behalf - how can any one person be certain of another person's mindset or experiences and/or how they will react to a given remark? Sure, speak up for those who are unable/too meek to speak up for themselves - but don't go into a full-blown forty-page rant about how those kinds of comments are wrong or offensive for x/y/z reason - especially under some kind of saintly pretense (read: delusion) of 'educating the world'.
However, it is never necessary to make derogatory remarks in everyday conversation - and personally I feel that it reflects badly upon the person who feels the need to make those kind of remarks (the amateur psychologist that I am - I cling to the belief that people who feel the need to make fun of others only do so to mask their own insecurities). I also don't like it when guys make derogatory remarks or sweeping statements about women - but it isn't as though women don't do the same of men as well...
Often at my place of work I'll hear some hatchet-faced woman saying that
"All men are the same!" (usually pertaining to men either being lazy or untrustworthy in relationships) and it really puts my nose out of joint because I'm not at all like most men, but I've learnt - especially with that kind of woman - that it's better to bite your tongue, roll your eyes and walk away.
Some people are so set in their opinions that they're not worth wasting the breath on.
Edit:
Quote:
"because I'm not at all like most men,"
Ooops; looks like I've got my own stereotyping issues to deal with there.
Perhaps it'd be better/more accurate to say "I'm not at all like this woman's perceived stereotype of men".