Many bishops have written pastoral letters or other writings that explain Church teaching on marital sexuality. Posted here are several examples.
Bishop Joseph F. Martino - A Pastoral Letter on Chastity
Bishop Nicholas DiMarzio - Natural Family Planning
Bishop Glennon P. Flavin - In Obedience to Christ
Pennsylvania Bishops - Questions and Answers on CohabitationPennsylvania Bishops - Questions and Answers on Cohabitation
Bishop Victor Galeone - Marriage: A Communion of Life and Love
Bishop Robert J. Baker - The Redemption of Our Bodies (PDF - 600KB)