Well, if no-one wants to marry your princes, just wait until they get on the "side-branch" (i.e. cease to be princes), then marry your own princesses to them. You shall have a "invisible spare princes", ready to take over should your main line die. Just watch their loyalty - they sometimes like to make things like that happen faster, like by means of a civil war!
Your king, even if he hasn't been married already, will wait just a couple of years before taking a wife from within a populace. Just don't get him killed before this happens - unless you have another prince to spare.
Personally, I have never had a problem with my line. Theory is that your line might die off within ten years (your king has only one son, and dies before his son matures). Practically, you have to be seriously struck by some ill-fate to actually see this happen. I always have at least two sons waiting for the crown. And should I actually have just one, then make sure there are married uncles to take over should the oddball assassin come through...
Bookmarks