Who else is she supposed to fuck afterwards? You're gone. You're too busy doing the same shit to another 10,000 couples.
She's not gonna feel anything from her former lover anyways, she'll probably do some deranged shit to get back to you by herself. That's the power of realisation i'd love to see be added to the story.
But others have already said, this is primarily a comedy, not a drama. Whoevers GF the MC fucks, she probably won't be so profanely corrupted as to refuse returning to her previous lover. But in the mean time, you'll still have access to her honeypot :3