CMNF stands for Clothed Male, Naked (or Nude) Female.
CMNF stories cover a wide range of potential fetishes. Most of the time, the woman is somewhat reluctant and embarrassed, since herein lies a great wealth of conflict - the thing which drives fiction. However, this isn't always the case - for instance, the story of a stripper, surrounded by clothed men in a strip club, is technically CMNF. In most cases, the emphasis on the female's nudity amongst clothed men is central to the genre.
In my CMNF stories, there is very little actual sex. Most of my female characters are ENF, or Embarrassed Nude Females, and the stories tend to focus on these women, who find themselves in embarrassing situations, usually naked and in the presence of men. Escaping the situation, covering up and hiding their nakedness, is usually foremost on their minds.
There is occasionally some light bondage and spanking, but my CMNF stories tend to be a little milder than the reverse - CFNM, or Clothed Female, Naked Male, where domination and humiliation are more common.
As stated, my favorite CMNF stories tend to be the ENF variety - both to read and to write. Most of my stories will follow that theme. Perhaps a woman loses a bet, or loses at some strip game. Maybe she gets locked out of a house or hotel room naked, or otherwise finds herself out in public without a stitch of clothing. Occasionally, she may be tricked out of her clothes, or have her clothes stolen, such as while skinny dipping. And rarely, she may through a dare, lost bet, forfeit, or other reasons, find herself submitting to a spanking, forced to perform oral sex, and other similar things.
Sometimes, the female character grows to be comfortable around clothed men while remaining naked - most often, she tries her best to escape and find some clothes!