On a busy night last week at the Target on Atlantic Avenue in Brooklyn, Ari Nagel, 40, emerged from the men’s bathroom looking a little flushed and quite pleased with himself. “It’s better when it’s fresh,” he told them. “It” is Nagel’s semen, and it’s in demand. The 6-foot-2 CUNY Kingsborough math professor has served as a sperm donor for dozens of locals, siring 22 kids over the past 12 years with 18 women of various backgrounds.