Marriages thrive when spouses specialize in gender-typical roles.
If same-sex civil marriage is institutionalized, our society would take yet another step down the road of de-gendering marriage. There would be more use of gender-neutral language like "partners" and--more importantly--more social and cultural pressures to neuter our thinking and our behaviors in marriage.
But marriages typically thrive when spouses specialize in gender-typical ways and are attentive to the gendered needs and aspirations of their husband or wife. For instance, women are happier when their husband earns the lion's share of the household income. Likewise, couples are less likely to divorce when the wife concentrates on childrearing and the husband concentrates on breadwinning,