One of the finest discussions of this question is given by Germain Grisez in his book, Living a Christian Life, Vol. 2 of his The Way of the Lord Jesus, pp. 721-737. In Love and Responsibility (1960) Karol Wojtyla has some helpful things to say, particularly in the final chapter on sexology. I will first summarize material from Grisez, then from Wojtyla, and finally offer some practical suggestions.Read it all here
Thanks to Sue Cifelli for sharing.