Would any straight audience touch the bi content anyway? I romanced zev once because I genuinely enjoyed the character
I did it once and just out of curiosity about his arc. And leli just ninjamanced me.
And the only reason I managed to romance the male characters in da2 is because I can safely headcannon that they're straight. For example even if I only romanced anders once. I did not have to hear about his relationship with karl. And again fenris because nowhere in the game it was implied.
Bi characters like Leliana and Zev are pretty popular romances.
Do you mean same sex romances? You bet. There already plenty of straight posters, men and women both, that have done the s/s romance stuff.
And why would you need to headcanon that? If they were loyal to you, should it matter if they liked boys or not? Us bisexuals can and do have monogamous relationships all the time. 
Anyway, Merrill and Fenris were both stated to be ambiguous by WoG, with Anders confirmed as bi no matter what. The relationship with Karl always happened, the writers just decided that Anders wouldn't be comfortable telling a femHawke. Which was silly, given the attitudes of Thedas about sexuality.