The hospital donation center

A man and a woman were waiting at the hospital donation center. Man: "What are you doing here today?" Woman: "Oh, I`m here to donate some blood. They`re going to give me $5 for it." Man: "Hmm, that`s interesting. I`m here to donate sperm,myself. But they pay me $25." The woman looked thoughtful for a moment and they chatted some more before going their separate ways. Several months later, the same man and woman meet again in the donation center. Man: "Oh, hi there! Here to donate blood again?" Woman: [shaking her head with mouth closed] "Unh unh."