Mammary intercourse involves a man kneeling or sitting on a woman's stomach or chest, placing his erect penis on her cleavage, and rubbing or thrusting while the breasts are squeezed around the penile shaft, by either the woman or the man, creating tightness similar to masturbation,[1] and in simulation of penetrative sex.
Alternatively, the woman can tighten her breasts around the penis and move them back and forth.
However, Alex Comfort has said that mammary intercourse can produce orgasm in women with sensitive breasts (what Margot Anand terms local orgasms of the breast), and it was one of the nine substitute exercises for penetrative sexual activities, as detailed in the Paradis Charnels of 1903.
Since mammary intercourse is a non-penetrative sex act, the risk of passing a sexually transmitted infection (STI) that requires direct contact between the mucous membranes and pre-ejaculate or semen is greatly reduced.
HIV is among the infections that require such direct contact and is therefore very unlikely to be transmitted via mammary intercourse.