Marriage is by nature exclusive, just not when it comes to the one thing that most characteristically distinguishes it from all other relationships

"we are being asked by the gay or the women's movement. . . . to do three things, one [(no. 1)] compatible with tradition and two [(nos. 2 and 3)] not:
     "1. We are being asked. . . . [to agree that]
     "3. Life inside marriage is not to be construed as forbidding sexual relations with other persons. . . .
     "I do not call into question the teaching that marriage is a lifetime commitment, that its nature is exclusive, and that it is based upon fidelity.  I wish only to question whether these concepts are rightly understood when they are taken to refer primarily and necessarily to sexual congress.  Is it not instead the case, both in theory and in practice, that the validity of a marriage is judged by the intent and consent of the partners?  What counselor, when hearing a case of adultery, would judge by the act committed rather than by the agent's testimony of love, concern and willingness to preserve the married relation?  In these matters, to be sure, there is an important aspect known as 'injury'; but this is not an absolute standard unless the 'injured party' is of an absolutist persuasion.  Wise counselors learn that the greatest threat to marriage is, in fact, absolutism.
     "Far more important to marriage than adherence to rule is open communication between the partners. . . .
"marriage is made for men and women, not they for marriage.  This is precisely the message of our friends in gay and women's liberation. . . .
     "The rise of homosexuality and bisexuality in society exerts a pressure on marriage that causes us [(i.e. the church)] to re-appropriate its meaning.  If this is done, we are free to recognize that the forms of sexual desire do not matter when compared to the dignity of persons and their capacity for trust.
     "The gay liberation movement. . . . forces us to consider policy.  The movement should have our thanks if it . . . helps us to see that sexual purality is the very scene and stage upon which are played out the dramas of love."

     Thomas F. Driver, "The contemporary and Christian contexts" (1973), in Homosexuality and ethics, ed. Edward Batchelor, Jr. (New York:  The Pilgrim Press, 1980), 19-21, italics mine.  ="Homosexuality:  the contemporary and Christian contexts," Commonweal 98, no. 5 (April 6, 1973):  103-106.