I think BWA did everything SWTOR? Good and the bad? I feel terrible for them that fans react so violently towards them (not saying you) because of that, because there's probably a somewhat acceptable dev explanation for why they had to make a whole 'nother planet.
Coding mostly. Also expenses.
It would have been really difficult adding a new companion to the game from scratch-- Shale, at least, had coding within DA:O already. And I can't really speak for Sebastian, but considering how he was lacking in some ways, I would say he had less input in the game as a whole.
It would've been almost impossible to add a new companion to a game like SW:TOR and have them react to everything that can happen and react to the other companions and the like. Giving them their own story, quests and area just cut down on everything that was needed. (Thus, 'gay planet'
)
It wasn't the team being arseholes, it was them taking the only option they could.
The real question is, why didn't they allow homosexual relationships from the get go? No idea on that one, but it might have something to do with Lucas Arts for all we know.