I may be in the minority here, but I'm thinking that Ennis and Cassie's relationship was sexual, with Cassie being the initiator, of course, and Ennis going along (just as he did when she wanted to dance and when she wanted a foot rub).
At the bus station, Cassie is clueless as to why Ennis broke up with her, but she's also angry. She can't figure out why they're no longer together, but she clearly wants to know why. If they had never had sex, Cassie would have been able to figure out that Ennis just wasn't sexually attracted to her. Even if she never understood that he was really in love with Jack, at the very least she would have known that there was no sexual chemistry between them and she could understand why they were no longer together: that Ennis just didn't find her sexually appealing. She acted much too hurt when she runs into Ennis for their relationship to have been nonsexual. In my opinion, that kind of hurt only comes about after sex has been thrown into the mix. It seems to me that Ennis broke her heart. She clearly had wanted to marry him. She was in love, and she can't figure out why she got dumped.
Perhaps another clue is the half-empty bottle of white wine in Ennis's refrigerator, the legacy of Cassie. I think Cassie and Ennis shared at least one intimate moment in that sorry little trailer.