CFNM is an acronym for Clothed Female, Naked Male. This is typically a mild BDSM (Bondage, Sadism, Masochism) scene in which one or more males are naked in the presence of women wearing clothes. What happens within these scenes is as wide as the imagination. CFNM can refer to a group of men in a public pool with no swimsuits while the women are suited, as was more common in an earlier era. Or, it can mean much more, such as women dressed as nurses, abusing the genitals of naked men. A very common scenario is for one or more women to tie a man to a bed and gently tickle his stomach, thighs, feet, neck, armpits, and maybe even his genitals.
The gay male version is CMNM.