Let's say that somebody was mistaken in a fellow-internet user's gender, and upon learning the truth, found that their opinion of that fellow-user was changed by that knowledge.

Is that sexist or not? Does it matter if it revised their opinion up or down?

As an aside, if they suddenly found out someone had a different race than they thought, would it be racist to mention it later in a casual online comment? To bring it back to the thread, is your thinking any different about gender? Why or why not?