Sunset Sill wrote...
For me, it's not so much being jealous as feeling... unnecessary? I'm, unfortunately, the kind of person who sometimes wonders, "Would so-and-so be better off without me?" Like, maybe X and Garrus would be happier together than either one would be with Shep (while the other is left FOREVER ALONE, so of course they're not as happy). Shepard is too awesome for this to ever be a valid way of thinking for her/him, though, so that's just my personal hang-up getting in the way.
It happened for awhile with Fenris and Isabela too--they seemed to be really into each other, so it jaded my Fenrismance playthrough a little bit: "If I weren't romancing Fenris, he would have had a happy relationship with Isabela, and she would've gotten that fine elf ass she was after, I'M JUST GETTING IN THE WAY OF THEIR HAPPINESS NUUUUU."
A huge part of what I enjoy in the Garrus romance is that it's a story about best friends discovering that what they need/want has been right there with them for a long while now (er, minus, you know... 2 years coming back from the dead. right). I was extremely fortunate to marry my best friend, so I have a definite connection to that kind of story arc. So in an alternate setting, I can accept this happening with Garrus and another squadmate, if the foundation of said reality is that Shep/Garrus never, ever happened. (...perish the thought!)
With Fenris... yeah, that was much harder for me to accept gracefully just because he doesn't leave Isy for 3 years after they hook up. :/ I can come up with a number of reasons why that would be the case, but the initial awareness of it is something of a slap to the face. This alternate storyline with Garrus doesn't have that same undertone of insult to me.