So I have this sim who had a girlfriend, but was also romantic with another girl. Unfortunately, his girlfriend has seen it (him, his girlfriend and his other love interest are all in the same club) a...
Maybe they need time apart, like a small break. I say take one sim for vacation for a couple of days. Once they cooled off, and after some time the red in the bar may go down, you can try again. If after the, they still repell each other, I don't know what else could be done.
The idea of getting them flirty like someone else suggested may be the best option. I hope this all works out though, and I hope you get them back together.