Aedian: Alien Warrior. Alien Warrior, #3

Ten years ago, the Calphesians, a warrior race from far beyond our stars, invaded Earth with the intention to kill everyone they found and take the planet's resources for themselves. In order to prevent the destruction of the human race, a treaty was set, allowing the Calphesians to remain on Earth and use its resources as long as they didn't kill anyone or hurt the people. In addition, they would be able to pick and choose from a selection of human women to marry and have children with, in order to keep their kind from dying out. Every five years, three women are randomly selected and married off to the strongest warriors of the Calphesians, and this year Roxanne has been chosen.
She's angry and bitter about having her life decided for her, and nothing changes when she meets Aedian, her husband to be. He's boorish and crude, and treats her like she's insignificant most of the time, and all they do is fight. But when another race of aliens seeks to destroy the Earth and everyone on it, everyone will have to band together to fight them off, and both Aedian and Roxanne will realize there's more to each other than they'd thought.