Recognizing that behaviors which one thought were innocent might actually be hurtful and alienating to others is a very uncomfortable and difficult thing to do. Getting defensive is a much easier option.
However, without communicating it, there is no way for the former to know about it. And it needs to be explained directly, or right on the spot, if you really feel so strong about this particular issue. If it was indeed done innocently it should be no problem to sort it out. It is a little unfair to put everybody on the same boat without giving them a chance to see the situation the same way you do. And maybe the offended person will find out they didn't need to be offended, after all.
Anyway, general comments in blogs, while helpful to vent and rant, are usually not aimed at anyone in particular and then of course no one takes it personally. No one will think "oh, so that specific comment I made the other day, they thought it was offensive". It actually creates tension and eventually working with others becomes awkward and uncomfortable, because everybody is too busy trying to figure out what is appropriate and what is not.
Why do so many people fail to realize, while negative intent makes actions malicious, lack of negative intent doesn't make actions harm free?
A 90 lb dog can severely injure a human even if it's just playing, same for Everything you say, even if it's "not intended to harm"
To bring it back to the question of whether it's appropriate to respond to unwitting acts of sexism with anger: Depending on the particulars, it very well can be. Calmly talking about things is preferable in many circumstances where civility is valued at a premium, but it has limited utility in that it isn't particularly salient. Whereas a few harsh words can really grab someone's attention, and can show them that what they said was hurtful, rather than merely telling them. And, this being the 21st century, oftentimes the reason why someone might still unwittingly ostracize women is quite likely to be because, to put it bluntly, they're kind of dense. Meaning that harsh words might get through to them where soft words won't.
Done correctly, it can still be a compassionate act. Especially if the attention-grabbing jab is followed up by more patient, civil conversation. A little pain now in exchange for a lifetime of improved relations with the rest of one's own species is a pretty good deal, all told.