Because we are in the playersexuality thread and the main argument against it is the fact that they would be able to romance players of the opposite gender as your character in other playthroughs. It's the same thing. If you can go through a game as a female Shepard and never know that Morrigan is able to be romanced by a male without meta gaming and that's okay. Then it should also be okay to play as a male and never know that she's romanceable as a female as well without meta gaming.
I'm against the playersexual approach because then the companions seem less real. Instead of having predefined interests that the player character can't alter, the character instead shifts their romantic preferences based on the gender of the player character. Or worse, characters are gay or straight *until* meeting the player character at which point they surrender to the player character's epically awesome awesomeness and decide to switch teams.
I prefer a more realistic approach where characters are either straight, homosexual, or bisexual, and the player character is unable to impose his or her will upon them. That is what I meant by stating a few pages back that I thought ME3 did it better than DA2. Male Shep isn't going to charm Traynor into his rack, Cortez isn't going to forget about his husband while sleeping with FemShep, and Garrus isn't go to experiment with male humans. I thought that in a small way added to their characters, in that it made them seem more like real people rather than video game characters with a straight/bi/gay toggle.
I wouldn't just tie a companions interest (or lack of it) to the player character's gender either. Personality and how you treat them should also play a role. A rabidly pro-mage LI probably shouldn't be too interested in a romance with a rabidly pro-Templar protagonist who is burning mages at the stake, for example.
Obviously that might require more than one playthrough to romance certain characters, but I don't see that as a bad thing.