The explanation you're receiving is inaccurate. I have deliberately designed the choice so that if you dislike watching her fool around with someone else, the game rewards you by strengthening your relationship with her and keeps him off of her forever. If you let them continue, you get a tiny economic boost over time and make it harder for yourself to pursue a relationship with her. The only content you're blocking off by not letting her fool around with others is content where she, well... fools around with others.
Basically, the game gives you a choice and shapes the story according to your preferences, rewarding what you like while downplaying, or even removing, the opposite choice. Each choice gives a different, meaningful reward.