Below is "Adoration of the Lamb" by van Eyck (1390 - 1441), a Flemish painter of the Early Northern Renaissance, featuring a nude Adam and Eve. Painted around 1432, this was one of the earliest nudes of the Renaissance that wasn't portrayed in a negative light. Not erotic as such, though the rotund belly of Eve indicates either Eve being pregnant or her fertility. Either way, there is a vague association with sex.
The Northern Renaissance was very similar to the Southern Renaissance in the sense that it was a similar transition to artistic freedom and enlightenment, and a time when the services of artists were afforded by more than just the Church.
The Southern Renaissance was largely driven by the thinkers of the time (known as the Humanists) looking back nostalgically upon the art and writings of the ancients. Though the North didn't have these ancient texts and so their focus was mostly on religious thought, specifically Christianity. However, the focus of study of the last 1500 years had been Christianity, so what changed to bring about the Northern Renaissance?The Printing Press
One thing the Northerners had going for them was the invention in 1439 of a German goldsmith, Johanne Gutenburg, known as The Gutenburg Press. Up until then practically all literature was produced by hand which was incredibly slow making books and other manuscripts expensive, rare and a luxury only afforded by the rich or large institutions like the Church.
The invention of the printing press changed all that. Where it would normally take a day to produce a few pages of text, the printing press could print thousands in the same space of time. This made books cheaper and more readily available and resulted in education becoming more widespread, and allowed more independent ideas to be circulated amongst the masses. Work began on translating many volumes from Latin, which was considered to be the language of the educated, though not a language understood by most, so more people could make use of them.The Protestant Reformation
This rapid surge in education was the catalyst for the rise of free thinkers, and consequently the protestant reformation.
With the dawn of printing came the dawn of propaganda. With printed materials becoming easier and cheaper to produce, and with so many people wanting their ideas spread, it wasn't long before opposing thoughts began jostling for position amongst general opinion. The Protestant Reformation was almost entirely based on this and now doubt without the invention of printing, Martin Luther would have been just another noisy fellow on a soap box on the streets of Germany.
But back to the subject of erotic art. I believe the increase in nudes and erotic art in the Northern Renaissance was partly due to the rise of free thought, partly due to the Southern Renaissance, and partly as propaganda used in a similar way sex is used in advertising today. The Catholic Church and its supporters were fighting a loosing battle against the Reformists' printed materials, and so I believe that the sudden rise of the nude in art being portrayed in a more favorable light was to attract the masses back to the church, or at least keep their attention. The Counter Reformation of the Catholics eventually gave way to the Baroque Movement in art.
Erotic Artists of the Northern Renaissance
Cranach the Elder