An excellent and very educational series. About a newlywed couple who are virgins and their comedic ups and downs in learning how to have sex. Not really a hentai, though there are scenes which can be called hentai. Tastefully rendered taboo themes. A funny "how to" guide for sex illiterate and experts.
(Source: MU)

Fuuta is an average-looking salaryman who's never had luck with women. However, all that changes when he meets a strange girl through a dating agency. She's nice, but has an odd habit of always wearing masks on their dates. One day, Fuuta sees his mask-wearing girlfriend being attacked by (what he thinks are) thieves. Fuuta saves her from them, and full of happiness and gratitude, the girl confesses and proposes to him. The night they get married, she finally takes off her mask and reveals her identity: She's the super-idol singer Kirari who Fuuta admires! Thus starts their life as a married couple.

Kyoubei Kurokawa, university student, has an aimless life. Having run away from home to live alone he doesn't even have a place to sleep. One day he was saved from falling off a bridge by Asumi Nanjou, a mysterious girl from the same university, that takes him to her apartment to treat his wounds. When she finds out about his situation, Nanjou decides to let him live on her house, on one condition: he'll be an invisible man. Bewildered at first, he later finds out what she meant: she'll completely ignore his existence. Meet Kurokawa and discover the advantages (and disadvantages) of being an invisible man and unravel the mysteries behind Asumi Nanjou.

(Source: MangaDex)

From Tokyopop:
Zerodaimu Kyupura Pa Azalraku Vairu Rua Darogu (a.k.a. Zero) once stopped the evil demon lord with his incredible psychic ability, and in the process saved the world from destruction and earned the honorable and highly imaginative title 'The Man Who Stopped the Evil Lord.'
Now, he has accepted a position as a teacher at Psychic Academy, a school for gifted psychokinetic youngsters who have demonstrated incredible raw powers and want to learn how to hone their abilities. Among the student body is young Ai Shiomi, Zero's little brother, a somewhat meek boy who, despite parental prodding and his fraternal reputation, feels that his limited skills hardly justify his presence at the prestigious academy.
However, everyone else is convinced that he, like his celebrated brother, is destined for greatness. A lot of pressure for a boy just entering adolescence!