The word "chaste" comes from the Latin castus meaning "pure". Though it has always included sexual purity, the virtue we call chastity has come to be more associated with abstention from unlawful sexual intercourse when it entered the English language in the mid 1200s than the Latin root implies.
To be chaste does not mean to enter into a duty imposed against the will. There is no virtue in keeping a rule that one does not believe to be good and true. Chastity has to do with the heart, with reason, with the will, and with desire. It is the exercise of self-control for a higher purpose. It begins with desire and when chastity works well, the whole of the person is in united in this virtue to do what is good, right, and true.
It is not just that sex within marriage is lawful. Sex within marriage is to be chaste and pure just as those outside of marriage are called to purity of heart and life. It is here that we seem to have grown confused. The presumption is that whatever a husband and wife choose to do sexually is pure because it is lawful, that is, within the bounds of marriage. Such a presumption is indeed false. What is lawful is not always what is beneficial or good. Indeed, that is the point. What marks sexual purity within marriage has been replaced by sexual license that too often mirrors the distorted values and purposes of the world. There is no goodness in this. The Church is not merely concerned for what which is lawful. The counsel of the Word and the witness of the Church has always been for that which is chaste, that is, pure. This is the counsel of Ephesians 5 and its call to Christ-like love that creates holy respect. The Church's moral teaching is not about rules broken or kept but about the purity of love, desire, and life.
There are those who would suggest that opening marriage to homosexuals will encourage such purity but it is intrinsically impossible for this to take place. Purity is not some principle. It is the shape or order of relationships within creation and one of the fruits of redemption is learning again to find contentment and peace within that creative order for it reflects God's holy design and purpose. If anything what the Church fears is that abdication of the creative will of God in marriage will permanently change both the nature of marriage and what is means to love. Sadly, the Church has done much to encourage the false understanding of purity by making the concern legality. In addition, the wink and nod the Church has given of late to the heterosexuals who cohabit have created the issue of justice. Our witness to the world is not merely "no" to this sexual behavior or that. Our witness to the world is purity of heart, the chaste love that is learned from God in Christ, in which self and desire do not control or define us.
Sadly we have told ourselves and our children, if you cannot be chaste, be cautious. When we do this we automatically assign the higher morality to caution than to purity. True love waits but the emphasis is not upon the wait; it is upon the true (pure) love. Surely we will fail what Christ has shown to us but love does not abandon us to even our failures. Love redeems us, forgives us, and restores us that we might sin no more. It is this last phrase that we seem to have forgotten. We are forgiven only to erase any remnant of guilty feelings we might have and to assure us that we can do what we want as long as seek forgiveness afterwards. Our antinomian character flaw has led us down a twisted path that offers us little that is comforting or good. It is as if we Christians can only understand freedom in terms of a life of Rumschpringe or perpetual adolescence.
I think it is high time that we reclaim the fuller meaning of chastity. It is not merely about refraining from sexual activity or intercourse. It is about the desire of the heart, the heart created a new in baptism, so that Christ may live in us. Love is not license and we do not suspend the call to purity simply because a piece of paper makes the relationship legal in the eyes of the law. We proclaim a sacramental love which, though not sacrament per se, is sacramental in that it always points to the blessed union betwixt Christ and His Church, a union of purity, love, and desire. We have been redeemed body and soul; the transformation of the mind and shaping of the desire of the heart bear fruit in the set apart life. This is what we need to preach and teach. We have said "no" often enough that people know where we stand. It is high time we began saying "yes" to the noble endeavor of Christian life, rooted in baptism, understood as vocation, and manifest as those who are in but not of the world.