I played through as the canon maleShep then as a custom femShep from ME 1 and through ME 2.
On the maleShep I romanced Ashley despite initial negative reactions, figuring she'd have a cool story or something. Nope. So, especially after the treatment the writers gave her in ME 2, Shepard gives her the cold shoulder. The only problem is...who the heck can this character start a relationship with?
Bachelorette #1: Miranda. She's pretty, though dresses like a space-hooker and has an ego big enough to fill the Normandy 2's cargo bay all on it's own.
Bachelorette #2: Jack. She tries too hard to be "badass" and it comes off as trying too hard. I'm all for strong female characters, but she isn't strong, she's just the hollywood pop-culture generic image of "strong" and "misunderstood." I liked her story regardless.
Bachelorette #3: Tali. It kind of freaked me out honestly. You helped her through her pilgrimage in ME 1, and that's sort of a rite of passage, placing her comparitively to about 18 human years, I would guess. Shepard is about 30, I'm also guessing, being his/her current rank and service experience. Sure, Shepard was a marinecicle for two years, so now she's a little older. She's easily the most likable and level-headed of the three options and it's intriguing that Bioware hasn't allowed us to see her face yet. But...romance with a divergent amino acid based lifeform? That romance is destined for a room with seperate beds even if you get past the age difference.
With my femShep I debated Liara and Kaiden in ME1. I was tired of Ashley and blew her up this time around on Virmire (haha). I finally settled on Liara though since an alien love interest seemed cool to learn about asari culture and whatnot. She was kind of boring but Kaiden reminded me too much of Carth from KotOR (same voice actor), only alot cooler. I still am bothered by Liara's fake eyebrows though and the writers really seemed to flush the character down a toilet in ME 2.
Bachelor #1: Jacob. Could they of made him any more boring? Plus, as others have said, my femShep's dialogue was like she was in a bad porn movie around him. I was fully expecting a "nice boots, wanna f$&% ?" line at any moment. Jacob is a nice character but all the saluting and the sheer boredom of his personality made the romance implausible.
Bachelor#2: Thane. Thane is a fun character from an interesting race. His story is cool, and so is his loyalty mission. But femShep getting humped by a terminally ill space lizard doesn't do it for me.
Bachelor#3: Garrus. His personality is cool and would fit most closely with femShep's in a relationship, I feel. However...he's Turian, different amino acid makeup combined with what another poster mentioned of him looking like a giant cricket were both unsettling.
Summary: I didn't romance any of the ME 2 options with either my maleSheps or femSheps! I did make a maleShep infiltrator that wasn't an import from ME 1 and had him initiate the romance with Tali...but it just felt awkward still. In short, I thought the best chemistry for maleShep or femShep is with Kelly. Otherwise my Shepards just spent time watching their fish or the space hampster on it's wheel while wondering who replaced regular Liara with bizzaro-Liara.