A new survey of men who use online sperm donation sites sheds light on their motivations, preferences and experiences with donation.

Most are straight and donate for altruistic reasons, and one-third favor anonymous donation, researchers found.

Only 11 percent of donors are gay and nine percent are bisexual, but these men more often desire open-identity donation and would like contact with their offspring, the results showed.

A concern "about online sperm donation is that it is potentially open to anyone to become donors. By comparison, men who wish to donate via a licensed U.K. clinic have to undergo a stringent recruitment process, including medical screening," said lead author Tabitha Freeman of the Center for Family Research at the University of Cambridge in the U.K.

"A striking finding was the number of men - about one third of this sample - who were using the website to pursue anonymous sperm donation," Freeman told Reuters Health by email.

On the other hand, she noted, "By enabling direct communication between those wishing to donate and obtain sperm, connection websites enable donors to have greater contact with recipient families than clinics which is a key reason that some men wish to donate in this way."

In the spring of 2014, the researchers surveyed 383 men registered as sperm donors with Pride Angel, a U.K. based connection website for donors and recipients of sperm.

At that time there were more than 5,000 sperm donors registered on the site, although the site also registers sperm recipients, egg donors, egg recipients and co-parents.

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Donors who responded to the survey were an average age of 37. Almost 90 percent of donors whose donation eventually led to the live birth of a child were white, and they were more often employed full-time and had a university degree or higher compared to the overall population of donors on the site.

Eighty percent of donors were heterosexual, 11 percent were gay and 9 percent were bisexual, according to self-report. About half were single and did not have children. Most said they wanted to donate to help others or to pass on their genes, and having family or friends who had experienced infertility was also common.

More than half of the men preferred identity release, co-parenting or another arrangement.

Donor anonymity was removed in the U.K. in 2005 - children conceived using sperm donated after that year can find out the identity of their donor when they turn 18, Freeman said.

Women who conceive using donor sperm "may be interested in knowing more about the donor as a person because of the potential for future contact between their child and this man," she cautioned.

Half of straight donors said they preferred "natural insemination" compared to 26 percent of gay and bisexual men, as reported in Human Reproduction.

In general, there are many more sperm donors on connection websites than in brick-and-mortar clinics, Freeman said.

"Private donation arrangements without the regulatory protection of a licensed clinic raise a number of personal, medical and legal risks," she said. "It is therefore necessary that those using these websites seek appropriate legal and medical advice."

Some women may choose to find a donor on a connection website and carry out the actual insemination through a clinic, she said.

"There is a wide variation in the nature of these websites and the motivations of the men who seek to donate sperm in this way, and it is vital that there is further research in this area," Freeman said. "Although the findings from this research are reassuring in that the men who donated sperm appear to be well-intentioned and responsible, it must be noted that members of only one website took part in this study and the findings may not be representative of all online sperm donors."