@Machaka22: I have a different theory. Charisma (probably the best word I've heard anyone use to describe a harem MC in a LONG time!) is kinda like art; it's subjective. One person's charisma may be magnetic, but to someone else it's repulsive (I don't mean disgusting; I mean the exact opposite of the word magnetic). This is not limited to the characters in the manga, but the readers as well. The reason for this is it depends on a person's value, therefore what can make someone attractive is how it resonates with your values. So one has to ask: what makes a harem MC charismatic? For some, like yourself, this MC has little to none, but to the characters in the manga, enough to plenty exists to pull towards him.
Not in any way attacking you, btw. I just found your comment very interesting and enlightening in it's own way and wanted to add to it.