Haley's Story is very much a 'cest game. I do not know if that bothers you?
From the top of my head: (games where you exclusively go after a single LI are very rare, most games I mention are a bit of a compromise in this regard)
Melody is THE one to go, this one is purely in the ' girlfriend simulator' genre, the LI (the titular Melody) will not tolerate any side-romances after a certain point in the game, you will get a bad ending if you do not stay faithful
)
Price of Power haven't played it myself but as far as I understood this is also firmly in the 'girlfriend simulator' genre (one single LI)
Chasing sunsets a 'cest game where the main LI is your stepsis like Haley's Story but there is one other LI (it looks like the game will force you to choose between one of them. Not sure though, game is unfinished, as are most other games on this forum)
Acting Lessons bit of the same, the game basically forces you to choose between 2 LI's in the end (there is a third but that is considered a bad ending)
Summer heat A bit of an unknown factor. Very early in development. It might turn in a Harem game but I am not sure. A nice one to keep an eye on