An estimated 90% of men who have sex with men and as many as 5% to 10% of sexually active women engage in receptive anal intercourse. For unprotected anal intercourse with an HIV-positive partner with a . There are two main reasons for the effect of STIs on HIV transmission. Anal sex or anal intercourse is generally the insertion and thrusting of the erect penis into a person's anus, or anus and rectum, for sexual pleasure. Other forms .