How do Nymphs reproduce? They obviously just bang, but what do they bang?
In Greek myth, nymphs are the embodiment of a particular part of nature - a tree, a river, a rock, etc. They can be considered identical with the tree. So they don't reproduce in and of themselves, rather they exist as their tree exists, and cease to exist when it ceases to exist. New nymphs appear when trees drop seeds and sapling sprout, and disappear when the old tree dies. Their six (with mortals or with other spirits) is purely recreational (and symbolises the hedonism of nature versus the social constraints of civilisation).
To what extent that applies to TES, I don't know, but I imagine Tamrielic nymphs were intended to be the same as Greek nymphs when they were written into the games, and nothing in A Scholar's Guide to Nymphs contradicts this interpretation. They're the divine foundation of Mundus made manifest, I guess?