Something I never understood about Jessa’s character

When Jessa eventually starts dating Adam both the character of Jessa, and Lena Dunham herself, say that Adam had to initiate it because Jessa is not the type to steal someone’s boyfriend. But when her character is introduced in the first season Marnie says the reason she doesn’t like her is because she got with their friend’s boyfriend in college. And a lot of the first season is Jessa trying to break the cycle of being the other woman. So she seems EXACTLY like the type of person to steal someone’s boyfriend! Am I crazy?