the Doctor had just saved her life. She'd been really scared and the Doctor had looked after her. She's also probably not used to men not trying to take advantage of her. And she'd known the Doctor since she was seven and was probably really relieved to see that he existed.
just because someone is engaged to someone doesn't mean they can't have feelings for someone else.
Also, it's clearly established that she is experiencing confused feelings for both the Doctor and her impending marriage. And, there's plenty of examples of people who have gone to enjoy "one last fling" before getting hitched. One of the reasons that scene upset people was that she was clearly looking for a "one night stand", not a relationship.