A former Marine says he was denied a heart bypass operation at an Army hospital in San Francisco because he is infected with an AIDS virus. Retired Capt. Al E. Davis, whose 25 years in the corps included service in Korea and Vietnam, said he was told by a staff cardiologist at Letterman Medical Center that "the surgical department will not operate on you because it would be dangerous to their health." David said he was ordered to take tests after telling a surgeon he was gay, and the tests showed he was infected with the HTLV-III virus, which can lead to acquired immune deficiency syndrome. Officials from the federal Department of Health and Human Services said they were investigating Davis' complaints.