HeartFast. HeartFast Series, #1

They were the Guardians, sixteen special men and women with incredible powers who had sworn their lives to the protection of their galaxy, while upholding the laws of their world. It was a world that, eons ago, had suffered a devastating plague that had rendered more than half its population unable to reproduce. As a result, in order to assure the survival of their species, the HandFast law was enacted.
Every month, twenty couples were drawn by lottery. Couples whose sole purpose would then be procreation, the continuation of the species. For the length of one year, two complete strangers would follow the edicts of the HandFast, until the woman conceived.  Once that was accomplished, the couple was no longer bound to each other, and they could go their separate ways while the child was given to a couple unable to bear children.
It was a cold and impersonal law, but it worked. However, the Guardians were exempt from having their names drawn in the lottery.  The very nature of their jobs, risking their lives day in and day out to protect their galaxy, precluded any thought of bearing children. Which was why, when StarLight and Master Hunter heard their names called out to be HandFasted, their neat, orderly lives were turned upside-down.
Gone was the comfortable companionship they'd known. Gone was the brother-sister working relationship and the friendship. Now they would be forced into an intimacy neither had wanted, nor expected. Neither would they be prepared for the overwhelming passion they would find in each other's bodies, as well as in their hearts. And things would only get more complicated and dangerous.  Because their names had been deliberately placed on the lottery, and the drawing had not been a fluke.  Instead, it was the first step in destroying their world and every Guardian living.