I'm not sure, but it seemed to me that they set it up so that you specifically didn't romance anyone if you don't import a save with a romantic interest - I'd been told that there was supposed to be a portrait on your desk of whoever you'd romanced before, and I specifically looked for it and found nothing. Then they automatically kill off the one of the two humans who happens to be the same gender that you are. This makes me think that maybe the third game is going to set up a second-chance scenario.

I would've chosen Liara over him if I'd wanted to romance anyone in the first game, anyway. I've heard a lot of icky things about the Alenko/Shepard relationship - bad writing, misuse of gender stereotypes, power issues, and so forth. Enough to convince me that I don't really care to see it. (I've heard bad things about the Jacob/Shepard relationship, too, which is a shame. Jacob seemed like a pretty cool character, but I wasn't trying to hit on him.)
I've played the Thane/Shepard relationship, and I've seen most of the Garrus/Shepard relationship. In the latter the two seem to be on equal footing, which is always nice to see. And Shepard is pretty obviously dominant in the former, but not in a squicky, power-stealing way; he offers it to her, and she accepts it. That was my take on it, anyway, and part of the reason I liked it as well.