In an interesting twist, it seems that same sex relationships will be possible in Mass Effect 3, which is a switch from the earlier claim that no new romances will be possible. And no, Liara doesn't count as "same sex" because she is a unisex species.
Casey Hudson, Bioware executive producer of the Mass Effect series, Happy to confirm #ME3 supports wider options for love interests incl. same-sex for m&f chars, reactive to how you interact w/them in-game.:
It was originally possible for a male Shepard to romance Kaiden Alenko and a female Shepard to romance Ashley Williams in Mass Effect 1, but the option to was removed. However, hacked copies can restore the gay option.