What is implied is that someone who was free to pledge the nun gave permission for the marriage. You just don't know about that part.
Regarding Luther, you have to ask him directly ~ shouldn't be long now, eh? That nun is joyfully anticipating kicking you around the block.
No, that is not true in general; women or their parents certainly could ask their superior to be released from the monastic obligation for marriage rather than getting themselves smuggled out like fish. The parents, I hear, were around, too, in that particular case.
How do you imagine it: that anyone could go get a wife in a nunnery like we go to a supermarket?