Sundome!! Milky Way Vol. 4 (manga)

BIZARRE LOVE TRIANGLE

Yoshitake, the world's horniest man, has already landed the adorable and innocent Yuizono Haruka as his girlfriend. But the beautiful alien Lune still needs his virgin sperm to repopulate her interstellar home! She confronts him, accidentally revealing her true xenomorphic form, just as Haruka walks in! It'll take some tearful apologies, fast talking, and illicit fooling around for Yoshitake to end up with the best of both worlds- the love of a sweet Earth girl and planet-saving sex with Lune!