I'm not sure if it is or isn't untenable. I think what we're dealing with is a lot of smoke and mirrors on both sides. Rather than keeping things clear and comparing apples to apples, each is clouding the picture by putting their own interpretation on two different sets of statistics (number of divorces and number of marriages) and concluding that the amalgamation of the numbers support their position.
The Spedale report of a 10% increase in marriage (from 1990 to 1996) was not true, since the 2001 numbers (for Denmark and Sweden) showed a *decline* since 1990. Meanwhile, whatever is happening with the divorce rate, many are no longer holding off until the children are grown.
But it is interesting that gay marriage has made its greatest strides precisely where the institution is in direst straits. In essence the fight, such as it is, is over a corpse.
When the link between marriage and procreation is severed or even weakened, both suffer.