The Claiming of a Virgin King by April Andrews
When his father is killed in battle, William De Gulliame is thrust into the position of king. But armies of rebel aliens attack from the north, his own people in the south cower below him, and William is in increasing danger of losing his throne. He needs help and he needs it fast…and there is only one male who can give him that help, Lord Dargan of the island fortresses.
Lord Dargan has an army of his own, but he has never gotten involved in the war. Though an alien himself, he is settled in his fortress and has everything one male can ever need. So, when the king comes to him and asks for his aid, Dargan has every intention of refusing. Until he meets him that is, and realizes that the king has something he wants very much indeed…