Obviously the purpose is not to be friend with all the girls around
But... for exemple... to take my own choices, i planed to try to have sex with Rachel and Mrs Smith (the two most sexy ones of the game to my taste) but wasn't interested by Bella nor Liza. I was just surprised i couldn't help Bella.. just to help.
That's what would seem the most natural behavior to me, even if i'm not sexualy interested by her.
Anyway...
It was just to share my thought about it. I restarted the game without caring about Bella.