I think that Cassie not only is unaware that Ennis is gay, but also thinks that his indifference means that something is wrong with her. So whether Junior knows that Ennis is gay -- and David has a really good point, though maybe it's possible that she doesn't consider the possibility that he's gay, but she's somehow aware that he really isn't into women? -- that's not the point that Cassie gets.
And in a way, her dragging him onto the floor to dance could be another way that she keeps trying harder to be "the one."
By the way, did you notice how Cassie seems to get, hmmm, more drab or something as time goes on? I mean, Cassie's incredibly gorgeous and vivaceous the first time we see her, and she's got this spring to her (or maybe that's just the work she puts into her hair). But later on, her hair gets less curly, and she gradually looks less and less perky.
In a way, it's kind of like what happens to Lureen -- she gets less and less attractive as time goes by. And it isn't age, in particular... it's the way she becomes harder and more brittle.
