When the boy looks in the window, it is not simply the existence of "bad women" that he sees that upsets him and makes him question his view of the world. It is not as though he was unaware of "bad women" or that men sometimes drank early in the day or that they talked about things that were low and unclean.
The difference is that he sees the man he idolized only moments before, Jerry Tillford, doing those very same things. He sees Jerry as an integral part of the beautiful world of horses and particularly one who takes the same joy in seeing a horse run, in knowing that a horse has learned to be a powerful racer and something to be in awe of. He loves him because of the role he played in raising Sunstreak and the joy he saw in Jerry's eyes when Sunstreak was breaking the world record for the mile.
Then suddenly he hates him because he is low and mean and filled with lies, taking credit for everything Sunstreak did. Instead of being honest and clean like the horses and the world the boy knew before, suddenly Jerry is as changeable and low as the "bad women" and the inability to trust men as unchanging or a part of the world he enjoys so much shakes him deeply.