Here's what I did. I had my sim go to a place with a lot of foot traffic, like a cafe, and had her chat with 3 people that were good looking enough to be potential love interests. Then over the next couple of sim weeks, I had her spend approximately equal time with them, either on official dates or just hanging out. I had her chat, flirt, and kiss.
When looking at the green and pink bars, one guy was clearly not a favorite, so I asked him to "just be friends" early on. Within a couple of weeks, she had full pink and almost full green with one guy and only about 30% pink and green with the other, so then she became the girlfriend with the guy she "chose" and asked the other guy to be friends.
The only sad thing about this is that I liked the 30% guy more, but I had to give her what she wanted.
The guy she chose:
https://static.wixstatic.com/media/01c58f_bc8167e60c724f5abfc575dc8a0e12b1~mv2.png/v1/fill/w_740,h_457,al_c,usm_0.66_1.00_0.01/01c58f_bc8167e60c724f5abfc575dc8a0e12b1~mv2.png
The guy I would have chosen if it were up to me:
https://static.wixstatic.com/media/01c58f_6de0294bc6b44b24988f41e2303d4b3a~mv2.png/v1/fill/w_740,h_457,al_c,usm_0.66_1.00_0.01/01c58f_6de0294bc6b44b24988f41e2303d4b3a~mv2.png
