Sentence:  Since then , many Protestant denominations have made separate pronouncements , in which they not only approved birth control , but declared it at times to be a religious <WSD>duty</WSD> . 
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