I think that if you can interact with a Sim, you will have a "Romantic" category. If you have a "Romantic" category, but no pink bar, then choose some actions from the "Romantic" category. I think, it will take around three flirts, to get a bar to appear. Don't use the advanced ones too soon, or it may give you Negative Pinkery. A successful "Flirty Introduction", will also give automatic romantic rapport, with your target of affection. (TOA)