Intercourse by insertion of the penis between the thighs of the partner rather than penetration of vagina or anus. This was a common practice in homoerotic relations between older and younger men in ancient Athens since anal penetration was considered violating bodily integrity and humiliating for the receiving end (usually the younger man).

Needless to say, this method greatly reduces the risk of unwanted pregnancy and sexually transmitted diseases. It can get rather messy, though.