Wall of text incoming =D
To me I guess it has a lot to do with how I find people attractive in the first place. Sure, I have physical preferences as much as most people, but they're not awfully important. Some of the people I've found insanely attractive have not really been lookers at all. How they act is far more important than how they look. Heck, if I were to grade how highly I value different things that are of shallow natures, I'd put both voice (Garrus

) and smell above looks xD. And since I can't smell things in-game, nobody can stand out in that way as particularly attractive or unattractive.
There's no doubt in my mind that if I were to meet Garrus for real, he'd totally be able to woo me. He's extremely attractive, from that confidence he exudes, to the doubts, to his thirst for vengeance out of his love for his friends, and all the way to the clumsy nervousity he displays when Shepard hits on him.
And let's face it, it's not like turians look disgusting by any means. Alien, yes. Might not be physically attractive, but they're not disturbing to look at either. I find it easier to be confident in fantasies regarding turians that are just neutral in how I look at them, than I am with drell that are both attractive in the human kind of sense but also... not quite human. Thane feels less human looking than Garrus in many ways simply because he is closer to one. Does that make sense?
(no, it does not, cause I love the asari XD)
I also have this thing, well I guess I had it more when I was a kid, but it's like... I've always loved playing with the fantasy where I date an alien or a very life-like robot. People shunned by society even though they're absolutely great people, just judged for the wrong things. I'd love to be there for them. Suppose that plays into it, too.
As for why I don't like Jacob much, it's pretty simple really. It has nothing to do with looks for sure, since I prefer black men and find him very aesthetically pleasing and exotic looking (we don't have a lot of black people in Sweden so sue me ^^). It also has very little to do with HussyShep I think, since I imagine I'd sound just like her in the same situation =D
To me it's all about the line the game gives you that's basically not even in-character, but the game telling you "you can't progress talking to this character further as of now". Whereas Miranda tells you she's busy, Garrus says he's busy, Thane says he's still letting what you two talked about last time sink in (so a little bit freaky perhaps, but his memories work very differently from mine and it's not downright offensive)... and Jacob says he doesn't wanna talk to me?
The others -can't- talk to me. He -won't- talk to me. Yeah, down the garbage bin you go. Talking to me isn't supposed to make you feel we're "forcing talks", or something's just wrong with our chemistry, mister. And you're certainly not attractive enough for me to allow you to treat me that way. I know I should pretty much pretend this line of dialogue doesn't really exist, but I'm sorry, I can't. Ruins the entire thing for me =/