can someone explain how karen is ted’s first gf yet he slept with molly at 17?

like weren’t karen & ted together all through high school? idk how us high school work but im guessing when you are 17 you are in high school lol

anyway i’ve seen many people say ted cheated first at the freshman orientation party but wouldn’t he have cheated like way before ? idk im confused about the timeline probably ? or maybe karen started cheating before college ?