...Wow. You do realize that we actually see very little of either of them interacting romantically/sexually with anybody? Honestly, I see Cullen as straight, but him being interested in women does not preclude interest in other genders - we only have one data point for him, so pretending that that is a solid enough base to assume he is straight is not, in fact, the most logical conclusion, especially since the Dragon Age universe has so many bisexual characters already. The same goes for Cassandra, who, as far as I can tell, has even less of a precedent for heterosexuality than Cullen.
Never played the mage origin? He never shows interests in men at all in the first two games. And for Cassandra, the only thing, other than the anime, we have to go on, is something that honestly has no impact on the romantic side, and that is her "hero worship".
There is nothing to prove that Cullen and Cassandra have even the most remote of romantic feelings towards their own sex, so regardless of how many bisexuals there are, they have no reason to even be seen as anything other than straight at this time.