Tali going gay would be weird. Sorry, but it would be. Same with Garrus, Thane, ANY of the Korgan characters, Kaiden/Ashley, and Jacob (though Jacob is arguable).
_You want to know why? Because it would be awkward to have to look at dozens of rejoicing same sex gamers and have to reluctantly say, "...But that's bad writing."
It's illustrated with DA2, where in some characters were never shown as bisexual prior to the game, yet, because Bioware knows the option to choose your love interest in their games is such a big selling point, they made everyone in their mother bisexual. That's character inconsistency. That's bad writing (and kinda weird since it's not at all like real life, and considering the time period- actually scratch that last bit. Isabella is a female pirate captain in the Medieval Age. This obviously doesn't take place in any reality close to ours).
Garrus may never have said he wasn't into men, and neither did Tali say the same for women, but right off the bat you can see they're awkward enough pursuing an inter-species romance (especially poor ol' Garrus "Tight Ass" Valkarian). As for Kaiden, he already has a history of falling for the opposite sex, and Ashley is Christian. Jacob...well... I don't know, actually. Maybe you could make that work. And Thane was married. The Korgan are fertility-crazed, too, so I doubt they should even be an option to begin with (sorry Wrex fans :x ).
_Anyway, just know that altering an established character's personal preferences, and thereby altering their personality, so it can fit into the money-making "you can 'love' anyone you want" ideology is bad because it changes who they are. Plus, knowing Bioware, they'd make the newly added same sex romance options STUPIDLY OBVIOUS to where Garrus would be giving you the eye and Tali would be "accidentally" bumping her hips into yours. ...Ugh
Then again two different teams work on DA and ME so...
Not against same sex options, and if done right it's dandier than dandy! But giving fan service should only go so far. It was fine in ME2 because they wrote it to make sense and it wasn't too out of character, but any more alterations and you'll be changing characters in a direction that'll make them feel like totally different individuals (though Jack not being bisexual was odd to me...).
_You want to romance Tali or Garrus or Thane or the others? Go make an opposite sex Shepard! Shit, I know I did! I adore Tali, but who can resist Garrus? The option is there for more reasons than one, you know.