The novel mentioned how they met and got married.
She was a mercenary working for him or with him, and he was very interested and proposed to her. She wanted to reject, and he had several people not wanting it to go through, so they went through a hunting competition / rite to test it. They either tied in every way, or only saw animal couples, which lead to it being considered a draw and them being suitable. So they invited a few guests, kidnapped a priest to wed them, and done.