Aaaaah, i hope affiliation with the Chantry doesnt mean Cullen is straight too:( oh damn who i am kidding...
After thinking about it, I'm fairly sure that he is. Some might cite the precedent of Kaidan simply not feeling comfortable with stating his attraction to a male Shepard yet, or Anders being similar in DA:A, and both of those are true. But Cullen doesn't state his attraction to a female Warden either way; instead, he gets flustered and embarrassed and silly, and would clearly like to hide his attraction but is equally clearly unable to. For whatever reason, Cullen is simply not into a male mage Warden, and that leads me to believe with reasonable surety that Cullen is straight.