[The Mirrorball Man]

Originally posted by Mister Underhill:
his principal criterion for qualifying a book as worthy of analysis as an “adult” book seems to be that the work must include meditations upon/revelations about sex and/or religion.

Not at all. Read it again. What he wrote (and that's the precise point where I agree with him) is:

"Tolkien invented his own mythological world, but it lacks the dignity and the sinew of a real mythology, for it is without religion and essentially without sex."

In Jenkyn's eyes (and in mine), Tolkien's writings are not entirely convincing as a mythology (not as an adult book), because they fail to address fundamental parts of the human experience.

Sexuality and religion are two of these aspects, but there are more. There's not much sensuality in Tolkien's books, for example, apart from some powerful scenes involving nature.
Sure, there's some metaphysical content in the books, but he didn't really deal with faith itself, the relationship between the believer and the deity.
The Professor was fascinated with the corrupting power of evil, but didn't write much about the nature of evil. We just have to assume that Morgoth and Sauron were evil, but we don't know much about what was happening in their heads. In fact they seem to be more dangerous than really evil.

As you said, Tolkien's work established the conventions and archetypes for a whole genre of fiction, and you're right. But that's not what a mythology is about. A mythology should say something about our world, about our experiences, it should deal with the real world. The archetypes of Greek mythology are still used today in psychology or literary criticism. It's not surprising. They are relevant to our world, while Tolkien's achievement is different. He created his own world, with its own rules, and inspired generations of writers. But the universality of Tolkien's work is not mythological in nature, it's the universality of wonder.