Social Justice versus Moral Purity

Why is it that those who emphasize the importance of social justice tend to downplay the necessity of moral purity? And, why is it that those who emphasize moral purity tend to absolve themselves of being involved in social justice?

In the evangelical world, Christians who preach "no sex" tend to find little use for helping the poor. And, Christians who rally support for the marginalized often set aside strict requirements for sexual behavior.

Have you seen the same tendencies? And, if so, why do these approaches to "what's important" in our faith appear to be mutually exclusive?