To the dismay of gay-rights activists, the Food and Drug Administration is about to implement new rules recommending that any man who has engaged in homosexual sex in the previous five years be barred from serving as an anonymous sperm donor.

The FDA has rejected calls to scrap the provision, insisting that gay men collectively pose a higher-than-average risk of carrying the AIDS virus. Critics accuse the FDA of stigmatizing all gay men rather than adopting a screening process that focuses on high-risk sexual behavior by any would-be donor, gay or straight.

“Under these rules, a heterosexual man who had unprotected sex with HIV-positive prostitutes would be okay as a donor one year later, but a gay man in a monogamous, safe-sex relationship is not okay unless he’s been celibate for five years,” said Leland Traiman, director of a clinic in Alameda, Calif., that seeks gay sperm donors.

Mr. Traiman said adequate safety assurances can be provided by testing a sperm donor at the time of the initial donation, then freezing the sperm for a six-month quarantine and testing the donor again to be sure there is no new sign of HIV or other infectious diseases.

Although there is disagreement over whether the FDA guideline regarding gay men will have the force of law, most doctors and clinics are expected to observe it.

The practical effect of the provision – part of a broader set of cell and tissue donation regulations that take effect May 25 – is hard to gauge. It is likely to affect some lesbian couples who want a child and prefer to use a gay man’s sperm for artificial insemination.

But it is the provision’s symbolic aspect that particularly troubles gay rights groups. Kevin Cathcart, executive director of Lambda Legal, has called it “policy based on bigotry.”

“The part I find most offensive – and a little frightening – is that it isn’t based on good science,” Mr. Cathcart said. “There’s a steadily increasing trend of heterosexual transmission of HIV, and yet the FDA still has this notion that you protect people by putting gay men out of the pool.”

In a letter to the FDA, Lambda Legal has suggested a screening procedure based on sexual behavior, not sexual orientation.

But an FDA spokeswoman cited FDA documents suggesting that officials felt the broader exclusion was prudent even if it affected gay men who practice safe sex.