Sentence:  Pope Pius 12 , declared in 1951 that it is possible to be exempt from the normal <WSD>obligation</WSD> of parenthood for a long time and even for the whole duration of married life , if there are serious reasons , such as those often mentioned in the so-called medical , eugenic , economic and social `` indications '' . 
Meaning of the word marked with <WSD> tags:  the social force that binds you to the courses of action demanded by that force; every opportunity, an obligation; every possession, a duty"- John D.Rockefeller Jr