Cullen never states what his sexuality is. Having a crush on a female circle mage does not make him straight.
Neither does Morrigan, Alistair, Sebastion, etc. It's just assumed.
You never see people going around proclaiming their straightness. It's just not something people "come out" to in the same way that people who are bi, gay, lesbian, etc. do.
If Cullen was meant to be bisexual all a long, then I'm positive that they would have had him crush on a male Amell/Surana too, because why not? But he's not, he's only ever expressed interest in females. He doesn't even like the more masculine female races (Dwarves/Qunari). The devs decided this too when they decided to not make him bisexual. It just doesn't work for this character.
People really need to just accept this. I mean, I would love to romance Dorian and Cassandra with a female character, but I can't. Their characters were written in a way that only made them attracted to men, and I accept this and love them regardless. I wouldn't want them to turn bisexual *just because*.