You need to remember that Lisa's story is rooted in a vanilla setting.
She is a run of the mill reserved college lass who is very happy in a steady and comfortable relationship with a matching average joe. There is nothing remarkable about her personality in the beginning. And that is where PG passes the reins to the player.
As you will advance the story further, you can mold her anyway possible. And that is including breaking up with Danny and moving on to other LIs cause her character has now developed into someone who doesn't like him romantically anymore. But most of these possibilities are still under production. The game is less than half finished and still focuses largely on budding attraction to other characters. Danny still being a big part of her life actually makes sense and gives impact to her guilt of fooling around with others.
After-all, you just don't jump relationships that quick in real life either. Perhaps consider following the game's story from Lisa's perspective, rather than your dick's?
/jesting