No, disagreeing with a woman is not sexism. The problem is that men readily speak up to disagree with us, rarely speak up to agree, validate, genuinely encourage, etc. When all you hear is disagreement, when that is the only way an entire segment of the population will engage you, it is a real problem.
I am debating flagging this piece. I see no good coming of it and I am frustrated that this stuff gets upvoted and then verbally pissed on when better stuff gets ignored.