I created both sims, originally started out with Margot, who is gay, and couldn't find a partner, so I made Rosalie for her. I should note I didn't play Margot as an active household after creating Rosalie. But Whilst playing Margot getting her life set up she kept rolling wishes to got on dates with people, kiss them, flirt etc.
Once I created Rosalie I played her, having her meet Margot etc turns out I did a good job making them, instant attraction and compatible signs. Rosalie constantly has wishes for romantic actions with Margot, I think the day after going steady she rolled the wish to propose. But after she had Margot move in and I was playing her again she never has any wishes for romantic actions with Rosalie, actually the only wish she ever has to do with Rosalie is to go skating on a pond. It's frustrating but not the end of the world, I am not about to break them up or anything I just like it when both my sims wish to get married or at least spend time together lol.
I've played Sims 3 since launch so I am used to just 'forcing' things if I have to. But as I have gotten older I like to play games a little more inline with sims wishes. I will just continue to make excuses up in my head, currently I am telling myself Margot is just shy (although she doesn't have the shy trait) and submissive and is letting Rosalie lead the way. That way I can pretend everything is perfect.