There is no evidence that swallowing sperm is dangerous, unless the sperm comes from someone who carries a disease that can be sexually transmitted. Diseases that can be transmitted by semen include gonorrhea, chlamydia, hepatitis B, and possibliy the AIDS virus.Moreover, just having oral contact with the genitals (without swallowing semen) can transmit herpes, syphillis, and human papipollomavirus if these organisms are present. You should practice safer sex techniques, including oral sex with condom, until you are sure that your partner does have an active stage of these diseases.
