You will still be able to interact with any of the Main Girls that you've gotten up to Love level 4 (as well as Morgana) and any of the Side Girls that you've interacted with - although depending on your actions some may or may not be on your side to begin with. You'll also be able to interact with any kids you've had by the end of Act Two :)
"Shit's about to go down", though, as Cursed said, so expect a big shakeup in the game world once you're in Act Three. Probably won't have time to go to any festivals with Kana, for example... ;)