I don't see it as having validity insofar as it being the *only* reason for marriage. It would make more sense if he was stating that the criteria for marriage is being able to biologically produce a child.
However, that point would not hold up under scrutiny because there would be many straight couples who would be a marriage license or mandated to divorce once they could no longer produce children.
Rather, I think the product of a marriage is naturally cared for by the producers. It is a bit crass and basal as such because love for one's kids is beyond that, but it is certainly true, I believe without exception, that kids within the married family structure are well cared for. This isn't to disclude anything, just say that organic families have an exceptional (not without exception) ability to care for and safe-keep what is their own.
I think, that is where they were going. It happens naturally (well, per God's design in this world).
As such,there is a spirituality that atheists and agnostics can't deny. We are called to love one another as Christians. Everything in creation points to the love of/in a family unit, I believe, on purpose.
-Lon