"A 1980 study of 2,345 women done by Dr. Susan Johnson concluded that “the sexual behaviors of lesbians are associated with a lower risk of most sexually transmitted diseases“ (Johnson, 1987, p. 805).
Why? The answer is simple: the fingering, fisting, tribadism, and oral sex commonly practiced by lesbians do not move body fluid from one partner into another as efficiently as a penis ejaculating into an anus or vagina.
Considering these studies, and emphasizing that only five cases of suspected woman-to-woman HIV transmission have appeared in the scientific literature since the AIDS epidemic began in 1978 (Rich, Buck, Tuomala, and Kazanjian, 1993), many health activists point to other serious health risks. Each year around 45,000 American women die of breast cancer. Smoking contributes to over 420,000 deaths annually in the U.S. (Castlentan, 1996), while traffic accidents claim the lives of around 40,000 people. Each year over 6,000 people die from accidents on the job (Coleman, 1997); ten times that many die from work-related illnesses. These statistics can make the risk of HIV infection seem insignificant to the lesbian who has sex only with other women.
Woman-to-woman sex is relatively safe, compared to activities like driving, smoking, heterosexual sex, or working. This can lead the polyamorous lesbian to conclude that the more lovers a woman has, the more time she’ll spend in bed with women, and the less time she‘ll have for other more dangerous activities."
--- The Lesbian Polyamory Reader: Open Relationships, Non-Monogamy, and Casual Sex / Marcia Munson, Judith Stelboum