Here is a comprehensive backgrounder published in National Review.

It answers the following questions in detail:

1. Why focus on opposing the recognition of same-sex partnerships as marriages? Aren’t widespread divorce and single parenting the real problems?
2. Why worry so much about policy?
3. Why wouldn’t you want to recognize committed, monogamous same-sex relationships?
4. How would recognizing same-sex relationships as marriages hurt marriage?
5. Isn’t the fight against redefining marriage a losing battle?
6. Why limit freedom in the name of sectarian values?

Here’s the detail on number 4:

Recognizing same-sex relationships as marriages requires replacing one basic vision of what marriage is (in our law, and hence in our mores, and hence in practice) with another vision of marriage. The new vision is one that equates marriage with the much broader category of companionship. Companionate bonds have great personal value, but they can’t ground in a principled way the norms that set marriage apart.

To the extent that marriage is misunderstood, it will be harder to see the point of its norms, to live by them, and to encourage their strict observance. And this, besides making any remaining restrictions on marriage arbitrary, will damage the many cultural and political goods that first got the state involved in marriage. Here is a summary of those goods.

Real marital fulfillment. No one acts in a vacuum. We all take cues from cultural norms, many of which are shaped by the law. To form a true marriage, one must freely choose it. And to choose marriage, one must have at least a rough idea of what it is. The revisionist view would harm people (especially future generations) by distorting their idea of what marriage is. It would teach that marriage is essentially about emotional fulfillment and cohabitation, without any inherent connections to bodily union or procreation and family life. As individuals internalized this view, their ability to realize genuine marital union would diminish. This would be bad in itself, since marital union is good in itself. It would be the subtlest but also the primary harm of redefining marriage; other harms include the effects of misconstruing marriage.

Spousal well-being. Marriage tends to make spouses healthier, happier, and wealthier. But what does this is marriage, especially through its distinctive norms of permanence, exclusivity, and orientation to family life. As the state’s redefinition of marriage makes these norms harder to understand, justify, and live by, spouses will enjoy less marital stability and less of the psychological and material advantages that flow from it.

Children’s well-being. If same-sex relationships are recognized, not only will the stabilizing norms of marriage be undermined, but the notion that men and women tend to bring different gifts to parenting will not be reinforced by any civil institution. Redefining marriage would soften the social pressures and lower the incentives — already diminished these past few decades — for husbands to stay with their wives and children and for men and women to marry before having children. All this would harm children’s development into happy, productive, upright adults.

Friendship. Misunderstandings about marriage will speed our society’s drought of deep friendship, with special harm to the unmarried. The state will have defined marriage mainly by degree or intensity — as offering the most of what makes any relationship valuable: shared emotion and experience. It thus will become less acceptable to seek (and harder to find) emotional and spiritual intimacy in nonmarital friendships. Instead of being seen as different from marriage and therefore distinctively appealing, they will be regarded simply as less. Only the conjugal view, which gives marriage a definite orientation to bodily union and family life, preserves a horizon richly populated with many types of association and affection, each with its own scale of depth and specific forms of presence and care.

Religious liberty. As the conjugal view of marriage comes to be seen as irrational (“bigoted”), freedom to express and live by it will be curbed. Several states already have forced Catholic Charities to choose between giving up its adoption services and placing children with same-sex partners, against Catholic principles. Some defenders of marriage have been fired or denied employment or educational and career opportunities for publicizing their views. If marriage is redefined, believing what virtually every human society once believed about marriage — that it is a male-female union — will be seen increasingly as a malicious prejudice, to be driven to the margins of culture. The consequences for observant Christians, Jews, Muslims, and others are becoming apparent.

Limited government. The state is (or should be) a supporting actor in our lives, not a protagonist. It exists to create the conditions under which individuals and our freely formed communities can thrive. The most important free community, on which all others depend, is the marriage-based family; and the conditions for its thriving include the accommodations and pressures that marriage law provides for couples to stay together. Redefining marriage will further erode marital norms, thrusting the state further into leading roles for which it is poorly suited: parent and discipliner to the orphaned, provider to the neglected, and arbiter of disputes over custody, paternity, and visitation. As the family weakens, our welfare and correctional bureaucracies grow.

People on both  sides of this issue should be able to articulate the reasons for each point of view. The article is a good place to find the case for natural marriage.

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