I'm no psychologist but I do have severe mental problems so I guess that qualifies me to toss in my .02 . For men I think it's in two groups, either closeted gay men who are in simply denial or people extremely into the BDSM lifestyle that the usual stuff just wont do it for them anymore.
My ex was a female cuck. A woman who enjoys watching her lover (myself) fuck other women and many related acts. We did 101 messed up things threesomes/foursomes/ private sex clubs etc etc and if someone really wants to to know I'll share but I let a few girls fuck her in the ass for my entertainment if that counts.