Many thousands of women are expected to converge on the nation’s capital for the Women’s March on Washington the day after Donald J. Trump’s inauguration. Jennifer Willis no longer plans to be one of them. Ms. Willis, a 50-year-old wedding minister from South Carolina, had looked forward to taking her daughters to the march. Then she read a post on the Facebook page for the march that made her feel unwelcome because she is white. The post, written by a black activist from Brooklyn who is a march volunteer, advised “white allies” to listen more and talk less. It also...