Thanks for the clarification, Larian.
It's always good to have more freedom and options, but I wouldn't have a problem with playing with a male and a female character or even only with a female character in co-op.

Personally, I think it could hurt some other goals/intentions if you try to go away from the male-female combination of your original concept. I would prefer a dialogue system with NPCs that depend on whether the female or the male character is talking to a certain NPC. And I really fear that the overall story will suffer by breaking up the male-female combination.
So, please only allow other combinations in the game if it's not at the costs of other important parts of the game.