Definition of Marriage

Jennifer Roback Morse reviews David Blankenhorn's "Future of Marriage" here. They each take a stab at a definition of marriage:

Morse: "Marriage is society’s preferred context for both sexual activity and child-rearing."
Blankenhorn: "Marriage is socially approved sexual intercourse between a man and a woman conceived both as a personal relationship and as an institution, primarily such that any children resulting from the union are– and are understood by society to be– emotionally, morally, practically, and legally affiliated with both of the parents."

Neither of these is strong enough, and not true in all cases. Many marriages are not socially approved, such as when a female school teacher marries her student as soon as he turns 18, etc. Instead of "socially approved", how about "legal", "permitted", "allowed", etc? Those would be true of all marriages. And both authors act mystified about the connection between sex and children.
Here is what I would offer as the definition of marriage:

"Marriage is the right to conceive children together."

The one constant in Stephanie Coontz's "Marriage, a History" is that every marriage was allowed to conceive children together. And that definition fits both Blankenhorn's and Morse's definitions also.
And "conception rights" are very relevent today, since we now need to ask if we should allow same-sex couples to conceive children together. We shouldn't use a definition of marriage that implies that this is a seperate question, or that society can only "prefer" or "approve" but cannot actually go so far as to prohibit or allow. We need to prohibit same-sex conception, and preserve marriage as guaranteeing the right to conceive children together.