Ok. Let me see if I have the energy for this... first of all I would like to preface this by saying I agree with and respect everyone's opinions on here (and as a result of reading them all it's taken me this long to respond) and the reason I won't be quoting anyone in this mini-dissertation is because it would grow out of control in an exponential way, I mean, if I quote one statement, I'll ending up quoting them all xD
Also I will be using names because continually saying 'X' will confuse me throughout this explanation (especially if I use smileys like the above), so IF YOU DON'T WANT TO BE SPOILED DO NOT SCROLL DOWN.
The reason I do not care for the Garrus/Tali romance is because there is no basis for it in the lore of the series or within the characters, individually and together, themselves. It is not a contrast to a belief that Garrus and/or Tali should 'only be with Shepard' or 'members of their species' and it is not out of jealousy on behalf of my protagonist. It all goes back to the establishment of character arcs and the laws assigned to the universe Bioware has created.
Garrus Vakarian enters the series a disgruntled and frustrated, albeit respected, C-Sec officer who finds his patience ever-shortening with the red tape and regulations that he perceives as being inhibitory of his ability to bring justice to the Citadel. Prior to this, we can assume, he served some sort of militaristic or service-related position for strictly Turians, seeing as everyone is required to do so in that race at the age of 15 -- we are not sure how long exactly he held this position, although we DO know it was long enough for him to become (unless they start 'em young)... *ahem* sexually active seeing as his 'blowing off steam' with a fellow soldier was on a military ship. But by the time WE meet Garrus, he is a much different man, more mature, although still almost innocent and naive in his beliefs about the law and regulatory politics, but aged enough in his intelligence to recognize the higher level necessities of law enforcement beyond the grunt work of war. As is suggested by his character, Garrus isn't exactly a "ladies man" come ME1, not that we know of, anyway, but we can assume that, given his history as being a tireless C-Sec officer combined with his frustrations in doing so, that he had little time for the dating scene.
Come ME2, Garrus is drastically different from his ME1 self, which already varied significantly from his time serving in the military. He has been betrayed, seen his entire unit die under his responsibility and had to live with the survivor's guilt, in addition to trying to recover from having been thrown into as expansive and fatalistic a war as that against the Reapers, only to be thrown back into normal life after Shepard's death, what with the Council denying the threat in its entirety. It's pretty similar to soldiers trying to assimilate back into society in present day, no?
And as we see throughout the second game, Garrus is absolutely CONSUMED by his lust for revenge, by his frustration and self-deprecation to even open up to the person he respects most in the galaxy, let alone a 'lady friend.'
It is only AFTER his reaching closure with Sidonis and the betrayal of his men that he even considers becoming sexually active again, which wasn't even at the forefront of his mind -- seeing as he was only talking about a previous encounter, not projecting another -- until Shepard makes the move. And it is only then that he is able to consider it, seeing as he had been so entirely consumed by his grief, lividity, and vengeance.
And, consequently, he only even CONSIDERS it because it's SHEPARD. He feels he can trust no one else in the galaxy, that he has no other friends in the galaxy, and respects his commanding officer more than every other person in his life combined. If he is to allow himself to be vulnerable after so long a period of self-torture and discipline -- seeing as his 'blowing off steam' was a fling when he was younger and less haunted by the horrors of the galaxy -- how COULD it be with anyone but Shepard?
And thus, it seems to me as though, unless romanced by Shepard, Garrus would remain a single man for a prolonged period of time as he learned to cope with his traumatic experiences and matured to an age where he could MOVE on with his life, rather than dwell on it. This is something I feel Tali cannot offer him, and here's why.
Tali. I adore the girl. When we meet her in ME1, she is still considered a child by her own people as she has not yet completed her pilgrimage. That alone should speak volumes of her inability to support someone as crushed as Garrus Vakarian (though we all know she matures come ME2 and develops, this is not my point). She is naive and foolish but blessed with a genuinely good and kind heart, and an absolute gift for understanding technology (and as a Quarian I'm sure that makes her one of the most important people society can offer). This in addition with the fact that her father is a member of the admiralty board puts a tone of pressure and strain on her to measure up to what everyone expects of her, but, again, she is still only a child. And come ME1 when she is thrown into the massive conflict that is the imminent Reaper threat -- as harbored by Saren and the Geth (ANOTHER traumatic experience for her seeing as these are the very creatures that drove her people from their home world) -- she is ultimately forced to grow up in a much more condensed, high-paced fashion. Quarians usually have an extended period of time for their pilgrimage to procure whatever beneficial facet they can find for the flotilla, and have the fortune of experiencing life on the way on their own terms, in their own time. They are then assimilated back into Quarian culture gradually - relatively speaking - and transition from childhood to adulthood with relative ease.
But Tali? Tali went from a curious, explorative Quarian child searching for technology who was then attacked and SHOT while in possession of data she didn't fully comprehend at the time. She then dealt with the Shadow Broker, a crime lord on the Citadel, and the events of ME1 within the span of..oh, what, a year?
She definitely was forced to grow up at a rate unfair to any child.
Then comes ME2. Tali has matured, is leading her own unit, is recognized by her people as an adult, and a respectable one at that, as a result of taking the experiences that transitioned her from child to adult - no matter how traumatic, such as Shepard's death - and projecting that energy into feasible means.
And even still that's not enough.
We see on Freedom's Progress that she herself lacks the ability to lead, to inspire her team to follow her command (not that I'm blaming her, but she herself believes this to be true, and a character's self-perception defines their behavior). For all she's endured and accomplished, Tali's assumed 'immaturity' has not, in actuality, been alleviated, but transposed into a higher institution, such as during her trial when she is viewed as both nothing more than her father's child with the inability to make good decisions and a pawn representative of the Quarian view of the Geth. For all Tali has offered the galaxy, she still is not recognized as a viable member of society.
And as a result, Tali continues to search for herself. She yearns to know who she is, where she fits and what she can offer in the grand scheme of things, purpose only to be found in the arms of Shepard, who treats her as an equal, invites her unconditionally into his team, and instills in her the confidence she lacks in herself. Sure, Shepard saves her life in ME1, and, as her still being in the mindset of a child, would have an extremely heavy influence on her perception of him to the point of being paramount in the rest of her life, but it is Shepard's TRUST in her as a reliable equal that allows Tali not only to WORSHIP him, attempt to emulate him in every sense and admire him from afar, but relate to him on an equal level and come to care for him as an individual.
Garrus, having been so self-involved, could not inspire this same confidence in Tali for he is too busy wrestling with the frustrations in himself.
Which is why someone like Kal'Reegar makes perfect sense as a romantic partner for Tali, because, as seen in ME2, which has only about 3 minutes of screen time between the two, as opposed to the HOURS UPON HOURS of screen time between Garrus and Tali, he has complete and undeniable faith in her and her abilities. He praises her affinity for technology and scientific understanding, respects her as his leader and prioritizes her life as being more valuable than his, which gives her the purpose and sense of self-worth she has been constantly searching for. It is this kind of an individual that can help Tali in establishing the self-esteem never instilled in her by showing it to her himself. Besides, we only learn to love ourselves through the love of others as children (for instance, my mother suffered horrible abuse at the hands of her parents and lacks self-esteem because of it, having never been taught to love herself because they made her feel unloved, without value and worthless. Even though she just got her MFA in children's writing, she believes she is a poor writer, and I am constantly trying to remind her that rationally she is brilliant, but she can't bring herself to believe it because it was never instilled in her as a child).
So. All individual characterizations aside, why don't I like these two as a pair?
IF they were to get together, and that is a SERIOUS if, they both would have a hell of a lot of growing up to do before they could actually be compatible together. In addition to disliking one another at first, jabbing at each other and displaying a relationship more akin to siblings than romantic partners - even friends - they are both 'dextro-based' (which really doesn't matter all that much BUT--) which means that Tali's linking suits with Garrus would actually be MORE hazardous than another species, because they can suffer from the same infections (I can't remember where I read this -- I'll try to find a link for you guys!).
In addition, purely from a writing standpoint, there has been no evidentiary development of the two as a romantic party whatsoever in any of the games, and from what I've seen in the leak, the emergence of the thing is quite poor, and thus, I dislike this pairing because it feels inauthentic, unjustified, and contrived. And whenever such an event occurs in any story, it inevitably removes me from the universe, because I've encountered an implausibility within it, which renders me less able to relate to it. It's not about the universe being the same as ours -- Reapers don't have to exist, the Citadel doesn't have to be real, other species don't have to live within our galaxy -- for it to be relatable. But it has to remain true to itself.
Garrus and Tali as a couple?
Completely dismisses that verisimilitude of the ME universe.
**NOTE: This post sounded a lot more final and fatalistic than I actually feel -- of course I'll still play ME3 and love it to death -- but it helps me make my point more when using such drastic language xD
OK! All done, if you managed to stomach that entire thing, I applaud, appreciate, and adore you

Continue with the discussion!