SÃO PAULO—With “jewel-tone eyes,” blond hair and a “smattering of light freckles,” Othello looks nothing like most Brazilians, the majority of whom are black or mixed-race. Yet the “Caucasian” American cashier, described in those terms by the Seattle Sperm Bank and known as Donor 9601, is one of the sperm providers most often requested by wealthy Brazilian women importing the DNA of young U.S. men at unprecedented rates.

Over the past seven years, human semen imports from the U.S. to Brazil have surged as more rich single...