Everyone is saying how there is no character reason for everyone to be bisexual. But it seems to me that, for three of the four at least, their characters WOULD lead that way quite logically.
Merrill - Has been essentially cast out of her clan, and was emotionally neglected if not abused before then. She is now in a foreign place, with only a small handful of people to depend on. Is it really any surprise she'd feel affection to the leader of that group if kindness and affection were shown to her?
Anders - More than anything else, Anders feels he is personally at war with the Templars. A war that, until recently, he had been fighting completely alone. Suddenly, he has a friend. An ally. Someone to help him when no one else was willing. Someone brave enough to stand up to injustice, despite the opinion being unpopular. Isn't homophobia another injustice that the spirit inside him would compel him to oppose?
Isabela - Is Isabela. I don't think I need to say more.
So other than Fenris, who I have not met, tell me where the characterization for any of the companions would lead you to believe that they couldn't or wouldn't love someone of either gender. It seems to me that Bioware wrote their histories specifically to lead to this result.