One of the many ways in which same- and opposite-sex couples differ is on this thing called babies. Gays and lesbians can get children only after an enormous amount of effort and deliberate thought: through adoption, buying a baby from a woman (a.k.a. "surrogate motherhood") or artificial insemination. Babies don't just suddenly appear.
By contrast, the things that men and women must do to make sure they do NOT have children outside of marriage are difficult -- abstain from sex, have a shotgun wedding, use contraception consistently or have an abortion (in descending order of moral virtue, in my opinion). People won't avoid umarried childbearing in a society that says what same-sex marriage says: Children don't need mothers and fathers. Alternative family structures are just as good. Young men who are raised to believe that fathers don't matter to their children will not become dependable husbands and fathers themselves.
Marriage is our most basic social institution for protecting children. Same-sex marriage amounts to a vast social experiment on children. Rewriting the basic rules of marriage puts all children, not just the children in unisex unions, at risk. Do not expect boys to become good family men in a society of Matthews who believe, as they have been taught, that men are optional in family life.
Advocates of gay marriage are trying to persuade us that SSM won't affect anyone but the handful of gay and lesbian families. Don't believe it. Listen to Matthew, who has absorbed the message of SSM very well.
Fathers are optional. Children are resilient. Adults are fragile, and their emotional needs come first.
Maggie Gallagher is a nationally syndicated columnist, a leading voice in the new marriage movement and co-author of The Case for Marriage: Why Married People Are Happier, Healthier, and Better Off Financially.