He needs a new wife and you are useful and captured, so why not? You are the daughter of Prince Daemon, should your father fall you stand to inherit after him, thus Ormund decides  a marriage  is in order, it is not his first loveless one. He expects obedience, duty, submission, anything else will be crushed to oblivion after all this is just politics. He needs more leverage and you stand in line to secure the realm, a potential way to mitigate the war