There's really no reason for any character to be 'bi' or 'gay' or 'straight', obviously they can be, but it isn't necessary. If Liara wasn't part of an all-female race that is bisexual by nature, you wouldn't know that she was bi. The only thing that suggests that most of the characters people think of as 'straight' are actually straight, is because they're only available to one Shep or another, but that logic doesn't work either as Jack is openly bisexual but she is only available to MaleShep. Unless its really necessary there's no sense in ever defining a character sexually, that should be up to the player. For example, if you want to think of Ashley as straight thats fine, you can romance her as MaleShep and choose not to as FemShep, but if I play as FemShep Id like the choice to do so (and thanks to the mod I now have that ability, which is super fun) . There is no reason why someone should be 'forced' to think of any character as straight or not straight (other than Asari like Liara of course, since all Asari are bisexual, thats part of the game and not just about her personally) .
I think the Mass Effect series has done a good job of demonstrating how much more fun a game is when the player has the choice rather than having something forced on them. Sadly they don't follow their own model when it comes to the romances, they take choices away for absolutely no reason, and thats what needs to change. If in your playthrough Tali is totally straight then good for you, but in mine she isn't, and thats good for me. What would be even better would be if I could do ALL of the romance and not just part of it

. Thats what we want, just the option, don't make any characters bi, or not bi, leave that decision to us, we'll all have fun when we all can make the choices we want with the characters we like.