The Wrath of Ashes by Lilo Quie

Asha is a kind young man, the bastard firstborn of the earl of a once-thriving valley kingdom. As war has torn his country apart, the earldom has grown poorer by the year, and his father meaner. In a land where man loving man is relegated to the dragons and their lands, Asha vows to be celibate, to earn his keep and flee to live a better life anywhere but there. But when a letter arrives to his father telling him that a dragon comes to claim him—everything changes.

Mezerath, king of Sauria, the Sin’s prince of wrath, has heard his mate for the first time in his dreams. His mate cries in pain, but he wonders if it would be kinder to let him pass than to prolong his suffering. When his loyal bed servant demands he pursue his mate and end their arrangement, Rath does just that. He loves his mate, but he hopes he is strong enough to become a dragon, to live like one and to extend his life for as long as the kingdom needs them.