Wait, wait, wait. Before the MC says "I love you" to Carol, you have three choice-menus that affect the further dialogue. You have 4 different dialog endings. If you chose "I love you", then you should not think about other girls at all (if we compare this situation with reality). And blaming the developer for this is a bit strange. You had a choice, and you made it. At the same time, you can not discuss the topic of lying/cheating with Carol at all (wait to reveal your feelings). Or you can discuss it, but not say that you already love her. I don't force the MC to tell Carol that he loves her.
Either way, it's a game. And don't take it too seriously.