Every time Lisa has an opportunity to have sex with someone when Danny isn't around, she can move onto the Cheating route, regardless of what you chose during the Swim Meet event. If you choose for her to cheat, she leaves the Faithful route, if she was on it. If she was on the Sharing route, and you choose to have her cheat, then she stays on the Sharing route, but also moves onto the Cheating route.
For example, in my playthrough, I put Lisa on the Sharing route during the Swim Meet. I kept her on the Sharing route and didn't let her cheat after that, until the scene with drunk Viv. At that point, I had her get onto Viv's route, because that's the only way that Lisa can get onto Paul's blackmail route. As a result of that choice, both the Sharing and Cheating routes are active in my playthrough.