A NEW trend in the US is seeing men offering their services as “free sperm donors” in order to have sex with women who want to get pregnant.

The men, some of who are married and are doing it behind their wives’ backs, advertise their services online and fly all around the country to “naturally inseminate” women wanting babies.

One of the donors is 23-year-old Kyle Gordy, a university student who advertises himself as a sperm donor on his Facebook page.

He told US ABC News’ 20/20: “I don’t do any drugs. I don’t smoke. I don’t drink. I don’t drink caffeine. I eat only sperm-friendly food: wheat, brown rice ... fruit and vegetables.”

Mr Gordy, who says he has made at least one woman pregnant, posts childhood pictures on Facebook so clients can see what their potential progeny might look like.

Another donor, ‘Joe’, is married and has three teenage children. He claims to have fathered more than 30 children in his seven-year career as a donor and says his wife knows nothing of his double life. ‘Joe’ says he has slept with more than 100 women.

“I have a Clark Kent life. Then, I have the Superman life,” ‘Joe’ told ABC News. “People might want to have millions of dollars in the bank, and then, you know, some of us might want to have dozens of children out there.”

The men say the wannabe mums come to them because “natural insemination” is the most efficient way to conceive.

One of Mr Gordy’s clients is ‘Serena’, who asked not to be identified. She recently had sex with Mr Gordy at his home.

The 44-year-old said she was unable to afford the fees at a sperm clinic where vials of sperm can cost around $700 and are screened for disease and genetic defects.

“I don’t care if I have a husband or a man, I just want the child,” she said.