Bored Kitty [v0.1] [Voidsquad] [v0.1] is in Alpha but it's just one girl (right now). You can even lock her down the relationship. Not super difficult, but it's right up your ally it seems.
More Than Daughter can be played so you have no other options but his daughter.
Hi Emi is another one. It isn't my cup of tea, but you may like it.
Several Japanese games are one girl only.... Lost Life comes to mind.
Several games can be played so there is very limited interaction with other characters. There's one I can't remember right now that you can go into the settings and just cross out characters you don't want to relationship with.