Leliana is bisexual.
Dating her as a male or female does not change this fact.

Zevran is bisexual.
Dating him as a male or female does not change this fact.

Anders is bisexual.
Dating him as a male or female does not change this fact.

Fenris is bisexual.
Dating him as a male or female does not change this fact.

Isabela is bisexual.
Dating her as a male or female does not change this fact.

Merrill is bisexual.
Dating her as a male or female does not change this fact.

Josephine is bisexual.
Dating her as a male or female does not change this fact.

Iron Bull is pansexual.
Dating him as a male or female does not change this fact.

They are not suddenly straight by dating someone from the opposite sex. They are not suddenly gay by dating someone from the same sex. You are erasing their sexuality by saying otherwise.

This has been an important PA to the Dragon Age fandom.