What shameful experience does Winston relate in his diary? Why did he do it?

Winston relates an incident when he was visiting the Prole section of the city. He visits a prostitute. As he gets closer, he realizes that she is pretty old. She has no teeth and there is nothing pleasurable about the encounter. He feels sick and sad about it more than anything else. Still, he gives her $2.00 and goes through with the act. I think Winston feels dead inside. He wanted to feel something even if the feeling is repulsive: at least he feels. The Party sometimes looks away when Outer Party members have sex with Prole prostitutes. The Party concedes that the sex instinct in men is hard to control and there is little danger developing loyalties in the casual sexual encounter with a Prole prostitute.