I think the key problem is that while Cassandra is a terrific character with an interesting background and tremendous room for growth, as the only in-party romance for the "Straight Male" character she does not fit the criteria that those of that demographic tend to desire.
She's too feisty. She has a tough shell. And she is not feminine enough to fulfill their fantasies.
"Romance" in the sense that we get it in a game like this is *generally* a more feminine form of story telling. Women enjoy and value working through a character's issues, getting to what's under the surface. There's a whole book genre dedicated to this, and within that there are subgenres for stuff like sci-fi, fantasy, and paranormal. (I write within the latter two of these for a living.)
I chatted with my brother and my boyfriend about this to get a more balanced set of opinion. Neither of them are mindless idiots or in any other way prone to being offensive. What they both told me, having looked at Josephine and Cass, is that (for them) Josephine would be "out" because she would be a chore since she's not an in-party character. Cass, on the other hand, is too masculine and over-bearing. It's difficult for them to feel like bad-ass male heroes with her beside them because with her personality it's totally possible for her to overshadow them. Further, a character like Cass (a) does not visually appeal to either of them (both like women with longer hair) and when this is combined with the amount of "effort" required to romance a character their reaction was, quite simply, "Why bother?".
Whether it was right or wrong not to include a more traditionally feminine romance option in a party-based role is another debate all together and is one which I, frankly, have no desire to delve into.
I am always sorry to hear people aren't happy with the romance options when they care about that aspect of the game. (I wanted Varric, darn it. So I can, in my own way, relate.) But at the same time, I'm going to pardon that exclusion and focus on the fact that I'm going to need to play this game EIGHT TIMES to see how each romance plays out, because I find every option interesting.