I pretty much always marry my princesses off to my generals to improve loyalty and to add a bit of extra insurance for the royal line. I did once have my king die without an heir (recently) and instead of BOOM game over, a civil war erupted between two generals married to my princesses. This is the first and only time I'd seen this in years of playing the game. I actually remember doubting someone that posted about this once as well, it turned out they were right. It is very rare though (as with inheriting another faction's lands through marriage) and certainly not to be relied on.