Everyone is saying they're against the harem route, whereas I'm sitting here saying he's just against THESE people being in his harem.
He already has plans to add at least the office lady into the harem, and its implied his childhood friend will want to be with him at some point. It's not that he's against a harem, he's against forming a pact with a bunch of strangers he doesn't know, especially when he's tied to THIS vampire girl specifically, especially after all they've been through so far.