Actually, there are several possible throuples/triad arrangements planned(Carol/Yuki, Sara/Ashley, Nat/Sasha, Roxy/Nikki has been hinted at a bit too), and the Dev has gone on record saying he plans to make it possible to "have all the girls". (That doesn't necessarily mean that such an ending will make much sense, mind you, but he is definitely aware of the demand for the option.)
The reason there's no harem tag is because the criteria have not been met. It's possible to bed all the girls, but none of them know about each other. That's part of the necessary definition of a harem on this site.