It's a harem just like To Love-ru where Yuki and Sairenji likes each other but before they realized it the Harem boomed upon Lala's appearance.
Tonikaku Kawaii is not a harem because they sealed their love by marrying right at the first chapter and the other romantic interest(Bathhouse Eldest Sister) just gave up after knowing the fact that MC was already married.
Also, the setting of this series is kind of in the medieval period where mistresses and concubines were accepted while Tonikaku is set in modern day Japan. Although, To Love-ru was also set in modern day japan, it has extreme sci-fi logic that pretty much cancel the laws of earth.
BTW Thanks for the update!