Theatergoers will have barely settled into their seats at “Reasons to Be Pretty,” which makes its Broadway debut this week at the Lyceum, before they will be jolted by the profanity-laced rant of a young woman directed at her passive boyfriend all because he told a friend she had a “regular” face. The entire play hinges on this seemingly innocuous comment, which is why the billboard outside the Lyceum describes it as “a love story about the impossibility of love” written by “Neil LaBute, playwright and provocateur.” LaBute’s plays are, in fact, so provocative that some past audience members have walked out midplay or screamed out “kill the playwright” or slapped an actor’s face after a performance. And that makes a side of LaBute happy. “It’s part of my makeup,” he says, “to ruin a perfectly good day for people.”