The social gospel is what Jesus spent most of his ministry preaching, not the sex rules.  He was most concerned by how the powerful exploited others.  Matthew 25:31-46 is about how we treat others, not about sex.  It says enter the kingdom because when you were a stranger, you welcomed me, not because you used complicated NFP as birth control rather than the Pill.  I'm not sure where the Church's obsession with all things sex came from; I guess probably Augustine's personal hang ups centuries later.