Sentence:  But when it came to marriage and children, she was in, and he, at twenty-eight, was out. She had said something like, "It's time to take this relationship to the next level or end it completely." You don't want to be <WSD>drawing a line in the sand</WSD> in front of a young fighter pilot. 
Meaning of the word marked with <WSD> tags:  To lay down a challenge; to provide a test of commitment (often one which carries a high risk) to a cause.