Secret Couple: Cupid's Prank

The protagonist is a 35-year-old, unimpressive office worker, Fujisawa Katsuya (藤沢克矢). At his company, there is a junior colleague with the same name but different kanji, Fujisawa Katsuya (富士澤勝也), an extremely handsome heir to a wealthy family who is promiscuous with women. One day, Katsuya (克矢) tries to protect a female employee who is about to be taken home by force by Katsuya (勝也). After a fight, he falls into a river, and his mind is swapped with Katsuya's (勝也), who fell with him... Katsuya (克矢) ends up living with Katsuya's (勝也) wife, Reiko, who is still a virgin.

(Source: Natalie, translated)