Knight's Fork (God Princes of Tigron #3)

Knight's Fork is a futuristic romance which combines a traditional quest story (loosely based on the Greek hero Perseus) with a romance around the issue of genetics and the royal need to breed. A Princess from a "superior" race was married to an alien Prince as living proof of a political alliance. At the time of the marriage/peace treaty no one thought that the Prince would become King of his people, and that an heir would be necessary. After many fruitless years, the Queen (as she now is) realizes that her only hope of keeping the peace is to find a sperm donor from her own race who has the same eye color as her alien mate's, whom she can trust, and who is not too closely related to her. When he says "No!" the stakes are too high for her to give up.