A family requires a mother and a father.
No doubt about it - kids get raised better by two parents, not one. That's why marriage is such a crucial institution.
Think about it - marriage equality has had one benefit about which I bet we can all agree: Now that gays have the right to marry, there's a lot less clamor for alternatives to marriage like domestic partnerships. Domestic partnerships (DPs) represent, IMO, the true threat to traditional marriage. As DPs proliferated over that past 20 years as an alternative for gays unable to marry, they began to be recognized more and more as an alternative for straights too. Many companies' benefit programs have recognized both gay and straight DPs, and extended benefits and perks formerly reserved for married couples. But a DP can be dissolved at the whim of either party, with kids left in the lurch. A domestic partnership is a terrible substitute for marriage when it comes to encouraging intact, permanent families.
For years, I have (as an employee benefits professional) been touting marriage equality as the best way to strengthen traditional marriage by stemming the scourge of domestic partnerships as an acceptable alternative.