I'm female and I don't find Garrus attractive. None of my Sheps have (or will) romance him. Same for Tali. Perhaps it has more to do with how their characters were set up in ME1 (as good friends, in Garrus' case, and as a little sister, in the case of Tali) that I couldn't imagine a romance with either, but their physical appearance certainly does have a factor in that.
One of my Sheps very nearly romanced Thane, but didn't go through with the final scene before the Omega-4 Relay. He seems like a sweet guy, but he's a bit too sappy for my Sheps.
Jacob, well I'm afraid I'm one of those who hasn't been able to get past Shep's super creepy voice acting at the beginning of their friendship/romance. I hear it gets better as the relationship continues, but I just can't get past the beginning ... so I just don't talk to him at all.
So all of my femSheps have, ultimately, remained faithful to Kaidan.
My manSheps have played the field more widely, simply because the romance options have been more appealing to me. I have 1 m!Shep who has stayed faithful to Liara, but that was purely to see what LotSB was like. I have one who happy!romanced Jack, one who angry!sexed Jack then romanced Miranda, and then several (including my canon) who remained single throughout both games because he's in love with Kaidan. I'm attempting a playthrough atm where a m!Shep romances and stays faithful to Ashley, just to have the save game for any potential content in ME3.
So, in conclusion -- aliens don't really do it for me personally, and there is always a certain amount of me that goes into my characters, no matter how well I try to role play them. Looks play an important role in initial attraction, but ultimately it's about the way the character is written, how compelling they are to me, and in what context I see their relationship with Shep. This is why Kaidan is always my preferred romance option (for women and men, oh that he was romanceable with my m!Sheps), because he's attractive, but also has an interesting back story and brings interesting moral debates to the story; and it's also why, conversely, none of my Sheps are attracted to Jacob, because, as good looking and built as he may be, the context in which their early relationship is written is just too off-putting for me to pursue to any degree in game.