What was Cassie thinking asking that? It's like asking "do I look fat?" What possible response was she expecting from a girl she's just met for the first time?
I think that's the point, she doesn't think before she speaks. She's a very straight-forward, blunt, but predictable character. I don't think the choice of Linda Ronstadt's "It's So Easy" was a coincidence. I think Cassie is there for contrast to show how easy it was for people to start relationships back then, as long as they were straight. I mean she goes straight for Ennis, Ennis goes forward with the motions, obviously no REAL sparks but the next thing she's talking about is marriage.
People often confuse coincidence with fate, and this is a prime example. Ennis always drank there, and Cassie worked there. Their relationship was based on convenience. Jack and Ennis on the other hand, became friends first, got to know eachother, and when they realized what they had, took it to the next level. Did Cassie deserve to be lied to and led on? Of course not, but she was just another of many victims to Ennis' fear.
Cassie is pretty much Ennis' Lureen. They took charge, and the boys don't object because they think it's what's expected of them. I feel most for Alma, because her and Ennis were together before he met Jack. The realization that your husband is in love with another man is hard enough for her, but to also realize that your entire marriage was a lie? And people wonder why she became so bitter! And more power to her for letting go... The fact that Ennis would've continued the marriage even after he knew Alma knew is testament to his desperation to do what society expects of him.
But before I get any more off topic, Cassie just didn't know any better. And like Ennis, Alma Jr doesn't object, she goes with the flow because she thinks that's what's best for everyone.