The couple warming up their genitals by the fire was a rather unexpected detail in an otherwise mundane early 15th century painting (and one I'm surprised did not warrant a mention). I suppose it was a mundane act indeed.
Good catch. I think that's where one major artery lays (femoral artery, if I searched correctly). I am not a back sleeper but I was advised to put a hot water bottle between my legs, to stay warm when camping in the winter. Apparently that was already well-known in medieval times.