The question is whether or not they could have continued their marriage or physical relationship if Keiko was "stuck" that way.
It obviously wasn't a topic broached in the episode (for obvious reasons) but it *is* one to consider. What would the ramifications have been if Keiko couldn't have been brought back to adulthood.
Read a newspaper lately? There are child brides in the Middle East.
Much of the Middle East? Not a good example of civilization. That child brides exist there is hardly evidence or argument for anything. A lot of things happen over there that don't happen in other developed parts of the world. Even here with the FLDS groups you mention, it's hardly an example of anything other than scatter groups of people who do things. By and large, child brides is pretty damn rare and widely seen as unacceptable.
The point I was replying to made it seem as though child brides haven't been around since "ancient times." It's obvious from reading any newspaper or online source that child brides are around right now. The location where it occurs is unimportant for this discussion. Child brides exist in the here and now.
And please note that I'm not saying it's acceptable. I find it reprehensible, no matter when/where/who is involved.