Nearly half of the 18 donors at present providing sperm for Wellington's Fertility Associates clinic have put restrictions on who their sperm can be offered to, director John Hutton said.
Of the 23 women on the clinic's books who want donor sperm, 14 are single and another three are in lesbian relationships.
Professor Hutton said his gut feeling was that donors who put restrictions on who could use their sperm wanted to ensure that children had the support of two parents.
Some men also objected to their sperm being used to help women over the age of 45 to conceive and some women who have donated eggs had also imposed similar restrictions.
Meanwhile, a controversial ban on gay sperm donors for lifestyle reasons could yet be reviewed. The issue will be discussed at a meeting of the Reproductive Technical Accreditation Committee next week.