Invasion of the Alien Alphas Series by Rowan Stone

Ryan thinks he's just an ordinary guy...until the night he encounters a genuine flying saucer.Drawn up into the alien craft, Ryan quickly learns that he is anything but ordinary. He's an omega, his fertility prized by the alien invaders, and the alien commander has every intention of claiming him for his own.Before he knows it, Ryan's not only filled with desire for his new alien overlord, he's overflowing with his alien seed.

An alien armada has conquered Earth, and all human men are required to undergo Synthesis, the procedure which will transform them into fertile omegas. Dex is determined to defy the aliens' orders until he sets eyes on the medic assigned to perform the procedure. And once Dex feels Dr. Xethren's skillful hands on his body, he knows resistance is futile...

David is shocked to learn that, instead of being claimed by one of the alien invaders, he's been assigned to the Spawn-o-Tron. The peak of Alethan technology, the device is programmed to deliver ultimate pleasure while ensuring Earth's omegas are efficiently serviced.Initially intimidated by the machine's impressive array of appendages, David quickly comes to crave its mechanical touch, and before long his belly is swelling with the potency of the alien armada.