There is two dialogues in game files (that I know) where Cullen is in relationship with male Inquisitor. 1) Same as in the OP, Dorian says to Inquisitor that in order to romance him Inquisitor needs to broke up with Cullen first. 2) Late game romance dialogue between Cullen and male Inquisitor who are in relationship.
All characters don't have files for both genders cause they weren't intended to be bisexuals.
Cullen was intended bisexual in some part of production, but devs changed him to straight. There is speculation of different reasons why it was changed, one being lack of time and resources (former employees tweet?) and other that it didn't fit to his character (Gaider). Gaider has also confirmed that he's straight in DAI.
Arguments about Cullen being bisexual don't fit his personality are most likely coming from stereotypes what bisexual guys are like and what they aren't like. What happened with sexuality doesn't affect personality?
Not suprised though, people think tomboyish girls are lesbians all the time and manly guys can't be gays etc.
Cullen flirting with female Warden makes assumption that he's straight, but that isn't still confirmation of his sexuality. Same with Anders and Kaidan. People are different, might have preferences, might "come out of closet" (if that is still used) later in life etc. In Anders case I at least know that Gaider who write him in DA:A has said that he wrote him bisexual, not straight. So his sexuality didn't change between games 