Sex, marriage, and relationships are one of the main areas of conflict that we as humans encounter. Sex and relationships are important to us. Why is it that so many movies and stories involve sex and relationships? Because drama is built on conflict and one of the main areas in which we have personal conflict is around our relationships.
So unless we believe that God doesn't care if we treat people well, unless we believe that he doesn't care whether or not we suffer: Yes, God does care about sex and marriage. He cares about it because one of the main ways that you personally can either make others happy or make their lives miserable is your treatment of your family and loved ones.And one on papal infallibility as well.