If you didn't get a girl to Love 4 before the Act Two finale then you won't be able to get them higher in Act Three (the exceptions being Morgana, who doesn't have a love meter anyway) and Chastity (who was a Faction leader and thus unapproachable in that way).
Basically, you should have everyone except for Morgana and Chastity at Love 4 by the end of Act Two or there will be negative consequences :P