I am caught up with the scanlation and they are caught up with the official release. I never touched the Web-novel / LN:
Spirit Migration counts as a romance but none of the reasons are due the mysterious protagonist. He only possessed animals or monsters to get around, as of now, and may or may not be human / a ghost. He indeed interacted with more women than with men, as of now, but none were love interests and these women haven´t even formed a stable supporting cast as of now. None of them even managed to figured out his true nature.
I would also need to give Berserk the harem genre by that logic... and Gate - Jietai Kare no Chi nite, Kaku Tatakeri (i agree) or Rakudai Kishi no Eiyuutan (i probably don´t) are locked down as people tried to apply harem, so why here? I removed it from the manga version and it always comes back within the day. I thus now have to ask the community:
Did the novel´s protagonist get a harem of at least 3 women at the same time, who are romantically in love with him, by the end of the web LN? I will shut up if that is true but i would prefer if the manga is kept as just a romance till such elements actually show up.
None can see the future and i would even apply the Harem myself.
The manga is a webcomic so i have no idea which demographic applies and i only know that it / the site should be male targeted. Shounen seems fine but the original release doesn´t feature furigana, so who knows.

I also read EU/US comics and am a librarian.
Manga-Masters, My ANN-Lists + Imdb