Yet is this dynamic flow inevitably sexual in nature?
This is the moot point for psychoanalysis.

But let us look at sex, in its obvious manifestation.

The _sexual_ relation between man and woman consummates in the act of coition.

Now what is the act of coition?
We know its functional purpose of procreation.

But, after all our experience and all our poetry and novels we know that the procreative purpose of sex is, to the individual man and woman, just a side-show.
To the individual, the act of coition is a great psychic experience, a vital experience of tremendous importance.