Ythree Maids a-Milking; as I have understood the tradition is a simple commentary on village life and the basic modesty / virginity versus "the need to prove a (particularly woman's) fertility prior to formalisation of a relationship - marriage.

The village morals worked this way: The basic understanding was that a girl would keep herself 'chaste' and be 'untouched' on the night her new husband took her to his bed. However (there's ALWAYS a however) - given that the security of a family lay in it's ability to produce offspring who would care for their parents as THEY aged and themn went on to have children of their own to aid them AND their (by then ) grandparents. The ability to bear children was also paramount given that infant mortality in the 19th Century and prior (among villagers) was better than 30% - sometimes greater

Thus; if a young couple were sen to have an "understanding" and the families were in agreement then, said couple were allowed a little "leeway" to experiment creatively. So when a lass DID fall pregnant (with a "Makings of a young cuckoo") it was seen as confirmation that neither she nor her prospective husband were barren and the wedding was thus arranged post-haste.

The reference to Drinking up the sun and drinking down the moon was basically a reference to the person involved going off on a 'bender' - either of regret, celebration of the conception or as an accompaniment to achieving conception. The inference being that they had drunk all night (or were prepared to) and may as well continue all the following day.