cuckolding
Cuckold \Cuck"old\, v. t. [imp. & p. p. Cuckolded; p. pr. & vb. n. Cuckolding.] To make a cuckold of, as a husband, by seducing his wife, or by her becoming an adulteress. --Shak.
I wrote some stories for a Cuckold site, bascially the husband knows of his wife's cheating ways and gets off on it. The fact that the other man might be better looking, more well endowed ect is another turn on. Humiliation plays a part as well. At least, with the site I wrote for.
The top niches that are being ordered at my sites are
1. Lesbian
2. Anal
3. Girls in Glasses
of course they change frequently
