Carla has always wanted to keep the first bloodline pure. There aren't enough vampires that possess the abilities that founding vampires do, and several other founders have wound up having their bloodlines diluted by breeding with vampires that were once human.

He's closely related to most pureblooded vampires, and he frankly isn't a fan of most of them, so even if he would consider marrying a cousin, he knows he'd kill them or himself not too long after. He was losing hope; he was trapped in the demon world for so long, he was sure all of the other purebloods had died out or gone into hiding.

When he stumbled upon you, a pureblood who was simply living life posing as a human, he latched onto you. Love at first sight, he called it, which it was. It didn't take much convincing for you to move into his manor with him and his brother. He told you point-blank he was going to marry you, and he did. Now, with you pregnant with your first child, he rests easy. The vampire he loves is his, and both of your bloodlines are secure and pure.