The term was developed in Usenet newsgroups when individuals pointed out that the popular term BDSM applied to only a fraction of the community. Some people, such as those solely into infantilism or cross-dressing do not get involved at all in the bondage, D/s or Sadomasochism scenes, and as such reject the label BDSM, but still consider themselves part of the alternative community. Many also eschew the terms "Fetish" or "Kink".
So in an attempt to be all-inclusive WIITWD was developed, and has enjoyed some popularity.
The term is purposely vague and broad, and many are not satisfied with it for that very reason. It has been said that nearly any sexual activity could be included under the acronym. The general policy, therefore, is to use WIITWD only in rare cases where there is a need to refer to the broadest possible group.
The earliest recorded mention of WIITWD is in the Usenet group alt.sex.bondage by Steven Davis, Jan 9th, 1995, in a summary/critique of Jon Jacobs' theories about what was and wasn't "real" kink. Davis explicitly attributes the term to another poster named Bill Bohrer.