I welcome the concept, however as someone said earlier in this thread, there is a difference between believable and realistic. A believable game does not need to conform to the rules or history of our world, it only needs to provide suspension of disbelief and make sense enough within the world of the game.Believable harems would be set in the Arabic world or some Asian dynasties of the past. There would be considerable political scheming by the harem members.
Actually that sounds like a pretty nice concept for a game.
If the game is trying to claim that this is taking place within our world as it is now, it is not believable to fuck students and teachers in the hallway of school for instance. However a good game does not need to drop that concept, they merely need to create a world where that is possible which many games fail to do.
By this I don't just mean "Oh this town is just corrupted" or "suddenly Magic", but also "How is this possible to keep secret?" for instance. It could be that this is a world where the government is ruthlessly experimenting with behavior modification to subdue the population or it could be that this is a world where magic or powers run amok and this is one such effect. In any case it doesn't really take much to make a game more immersive, yet most just fail to do so.
Edit: Regarding my " this is a world where magic or powers run amok" idea: Don't just say it... show other things that are happening due to this. Make the magic or powers really be a part of the world.