From King v. Smith, 392 U.S. 309, 314 (1968), which challenged the Alabama "substitute father" regulation that disqualified children from welfare benefits if their mother cohabitated with a man, even if he was not obligated to support them:
The testimony below by officials responsible for the administration of Alabama's AFDC program establishes that 'cohabitation,' as used in the regulation, means essentially that the man and woman have 'frequent' or 'continuing' sexual relations. With regard to how frequent or continual these relations must be, the testimony is conflicting. One state official testified that the regulation applied only if the parties had sex at least once a week; another thought once every three months would suffice; and still another believed once every six months sufficient.That must have been a fun line of questioning.
The Court invalidated the regulation for defining "parent" inconsistently with Social Security Act.