Ann's the kind of girl you know would fuck a hobo convention if you left the room for longer than 5 minutes.
No one would regard Ann as serious relationship material. She's the girl you basically have fun with, knowing it will never break your heart.
That's why the MC is so open to her doing slutty stuff. She gave Chris her number while on a date with you ffs! Why would anyone think she's serious.
Also, the characters are young, learning the ropes with sex and relationships.
It's not like you marry the first girl you date, I feel bad for anyone who didn't spend at least 10 years ploughing the field before settling down with a woman you know probably shagged the Harlem Globetrotters on her Spring Break.
Am i right?