maybe she is somewhat nervous that if she just starts having sex with you, youll lose interest or think of her differently , or maybe she thinks you guys have something stronger than shes ever had since you said her other boyfriends kinda just went for sex etc..

i think she maybe doesnt want it to ruin what you guys have, she hasnt felt that way for someone this is my opinion. i am not sure how to change it immediately but maybe it will over a short span of time