Anders is definitely bisexual, Fenris possibly as well. Well, Fenris is debatable, but Anders is, without question. He tells you that "His first" relationship was with that Tranquil NPC during the chantry quest. That means if you're male, he's gay and if you're female he's bisexual.
If you romance Fenris, as both male and female he leaves because the sex reminds him of his master. That doesn't mean he's gay or bisexual, but it does mean he's had men before in the past.
Either way, while this type of thing isn't game-breaking for me, I think Gaider's being a bit obnoxious when he says things like "it's our world, the sexuality plays by our rules!" He pretty much said the same thing in a post about Leliana reviving if you had killed her in DA:O.