Um, because they have two less options, one per gender, than gay players if two are gay and everyone else is bisexual. Let's assume there are 6 LI options. A gay male or female player will have one more option, 3, than a straight male or female player, 2, will have.
All Bioware games have been this way, except in favor of straight players instead. Even Dragon Age 2. If you take that into account, gay players would still have less romance options overall from Bioware games.
When I read the threads for Cassandra or Dorian or whomever and see lots of fans who are crossing their fingers in hopes they can romance their favorites with a player character of the same gender, it really does not seem very "fair" to make those characters straight when there's already been a plethora of straight characters in Bioware games already.
I would prefer all characters be romanceable by all genders and races, but if they are going to put into 2 gay characters, I don't think it's so bad to make the others all bisexual when historically all straight players have had much more options throughout the Bioware games.