No, the park scene is tied to Lena and Ian hooking around and Ian not being with Alison when Lena returns.
If that scene happens, Lena and Ian will decide if they want to have a serious relationship or just a casual hookup (On some scenarios, Ian can suggest that they should experiment together).
Once in the park, they will get that scene if Ian decides to do so, and if Lena has exhibitionism or is a Stalkfap model (I think that it may need to film the dance with Stan for Stalkfap), the scenes will go till the end. (My fave of the update)
If they commit to each other... both can cheat as soon as possible. Ian can hook with CIndy if he is in her route, and Lena can do the same with Axel, and afterwards, playing with Seymour.