But it's not even "everyone is bi"! LOL
Fenris does not mention any experiences with men, not even to SerHawke
Merrill doesn't mention anything about her love life, at all.
Anders only mentions Karl if you play male Hawke
Isabela is the only known bisexual (we've known this since origins)
Each playthrough is unique and so is each Hawke. Whatever happens in someone else's game with a Hawke of the opposite gender does not affect one's playthrough. I know this "subjective" sexuality can be sort of confusing but all it comes down to is, if the companion has not indicated what their sexuality is or talked about relationships with individuals of the same sex, assume what you are most comfortable with (they are straight, bi, or gay/lesbian). It's an rpg for Maker's sake! If no info is given, then role-play it as whatever you want.
It is the most resource-efficient system and everyone has the same chance or romancing the companion they like the most.