Slave Dragon by Charlene Hartnady

Take one female in heat and a testosterone-filled dragon. Then cage them together.
What do you get?
A raging need and a ton of frustration.
When Earth Prince, Sand, is captured by a fearsome enemy and thrown into a cage with a human female about to go into her heat, he knows he’s in deep trouble. The next few days are going to be agony.
What does the enemy want? Whelps. Should they succeed, the children will be snatched away from them to an unknown future. Sand can’t let that happen. He can’t! He’ll fight this burning need with all he has.
Macy is terrified when she’s thrown into a cage with a dragon shifter. Especially when he looks at her like that. His golden eyes are filled with…desire. Lust like she’s never seen before ‒ and it’s all directed at her. It’s daunting. She has to keep reminding herself that he’s a nice guy. That this isn’t his fault. Sand is governed by his physical instincts. Impulses he’s having to fight hard. Her respect for him grows with each passing day. As they get to know each other, their interest in one another grows as well. Not a good thing when your captors want you pregnant so that they can steal your children. Not a good thing at all!