What is "The very purest form of birth control ever devised. Ever"?

SCIENCE | May 23, 2010 Families' Every Fuss, Archived and Analyzed By BENEDICT CAREY Researchers videotaped nearly every waking moment in the homes of 32 families. Did someone say stressful? "The very purest form of birth control ever devised. Ever," was when single researchers witnessed the feuds that families get into. Also mentioned (regarding working couples): "Mothers still do most of the housework, spending 27 percent of their [waking hours] time on it, on average, compared with 18 percent for fathers and 3 percent for children (giving an allowance made no difference)."