So from what I did find (damn, you're right, no one can seem to agree here), is the idea is a little misleading. Only the Alpha male and the Alpha female mate in a pack. So they're going to keep mating only with each other. Unless.one of them is replaced as an Alpha. Or one dies.
So. It isn't "till death do us part" so much as "till someone is better than you". Which...is all too human, really.