He Who Fights With Monsters. MC gets transported to another world and doesn't build a harem. Wizard With The Flower Blades. MC gets transported into another world in the default avatar of a character he was creating. For some reason the system got his gender wrong, and so he was genderbent to female when sent to the other world. He is not exactly the type to care about physical relationships, and thus isn't the type to enter into a relationship, let alone build a harem. I would love to say more, but I have no idea what to say as the only quantifier was "no harem". Here's two, though they're both English Origionals. I could have added a third, but felt it came to close to having a "pseudo harem", even if there isn't one in the story so far.
Good job! After I made my joke post, I went searching for one on NU... I came up with nothing. I couldn't even think of one story that has 'Isekai' + 'no harem'.
Epilogue Tales of Paul Twister series A Hero's War Kill no More The first novel is a bit special though Edit: I linked to wrong novel, it's supposed to be Epilogue instead Last Science
Reincarnated as a Sword So I'm a Spider, So What Youjo Senki Woof Woof Story Mondaiji-tachi ga isekai kara kuru soudesu yo raising children in another world My Death Flags Show No Sign of Ending