The Jargon File (version 4.4.7, 29 Dec 2003):

MOTOS
 /moh?tohs/, n.

    [acronym from the 1970 U.S. census forms via Usenet: Member Of The Opposite
    Sex] A potential or (less often) actual sex partner. See MOTAS, MOTSS,
    SO. Less common than MOTSS or MOTAS, which has largely displaced it.