I tend to agree with Wei Dai, and it seems to me that your analogy between the way people behave on the internet and the way people behave in person is flawed. To illustrate this:

The internet behavior in question: the blogger didn't care enough about you to find out your gender, but did care enough about what you said to comment on it, also not realizing that you would read the blog post.

Real world behavior that would be actually analogous: two men (more likely to be uncaring) are walking down a street in a large city. Two other persons pass them, walking in the other direction and speaking with one another. The two men overhear something, but it is difficult for them to be sure of the gender of the two persons. Then, one of the two men comments to the other on what they overheard. He uses whatever gender pronoun seems to him slightly more likely, even while knowing that there is a good chance he is wrong, and he doesn't care.

Note the real analogy here: the two men don't care about the two persons they pass, but are interested in what they overhear, and so say something about it. They have no reason to expect that the persons will hear what they say, so, in their view, it doesn't matter whether they are right or not.

Of course, people may well underestimate the probability that other people will read blog posts about them, so maybe they should be more careful.