I'm not going to force any characters to always be threesomes. You'll always be able to spend time and have sex with just that character.
Scarlett won't be the only girlfriend option, she's just the earliest. Different characters will have the option at different points in their stories. Scarlett has hers at level 4, some may have it at 5 or 6, some it may be an ending. No one is out of the running.
However, the characters will continue to get close to each other as well as the MC. The whole house is exploring their sexualities together, that's the whole point.
If you choose to continue playing, I hope you enjoy future updates.