An easy example is Selina given how her mechanics make it difficult to work with other girls. And the confrontation with Tashley implies though won't respond well to MC having a harem, it's a good guess they might be another short of skill tree hijinks. Since both the Harem King and Outnumbered achievements give a 6+ count, I wouldn't image a harem route will be too much bigger than that, even if the members are potentially more fluid out of the collection of girls.
And just like that, someone comes along with the sound logic and reasoning.
Selina is going to be a killer, her mechanics make that clear, but even still her sister hates her. Getting with one could easily lock out the other.
Speaking of her sister, she's a hardcore femenist, as is her roommate. They're not likely to share at all.
They're also good friends with the cheerleaders, where as the cheerleaders don't mind sharing. Will the femenist want to share, knowing MC is banging 3 other girls?
I was actually surprised by Tashleys jealousy in the recent update. But again it's showing a full Harem is a little far fetched.
I mean there's loads of combinations that could work, there are loads that couldn't, the skill tree is obviously important to building said Harem, but I feel there's a reason that achiement comes at 6+ rather than he full 18