Sentence:  E. B. Tylor's idea that humans have only one choice, namely either to <WSD>marry out</WSD> or to die out, has become the most commonplace functional explanation. The necessity to <WSD>marry out</WSD> is always a corollary to the prohibition to marry sisters. 
Meaning of the word marked with <WSD> tags:  To marry someone who is from another family; To avoid incest.