I can't come up with the exact quote right now, or even where it says it, but I do know one of the "tips" that comes up when you start the game warns about making sure your couples are suited for each other.

The fact is, that different "peeps" do have different personalities. If you pick couples who have the same interest in children, they will generally get along. You might try the desensitizer kit on one or both of them to see if that helps. Otherwise I just suggest forcing at least one heir, and moving on to the next gen asap.