Omega to Akuma no Sex Friend Jijou

Yuuma, the unpopular host, summoned the incubus demon, Airucus, to grant his wishes. The contract's condition, however, was for Yuuma "to give birth to Ai's child." Yuuma is forced into conceiving a child and accepts Ai's body fluids by receiving his semen. But despite the fact that he was unable to become pregnant, Yuuma's body was in excellent condition due to the special nutrients contained in Ai's semen.

Yuuma became clingy after that and began to beg for Ai's semen himself—?! Whether he gives birth to an incubus demon child or remains an unpopular host—it's a difficult decision!!!

(Source: Kadokawa, translated)