Hmm, I suppose Liara counts, but it isn't quite as straightforward. The Asari are monogender with no concept of gender differences. According to the wiki, according to Liara "male and female have no real meaning for us" and if asked says she is "not precisely a woman."

Now obviously, for all express purposes, they sound and look feminine and even use feminine pronouns, so I'm happy enough to say it counts as a same-sex romance, but it is pretty clear they tried to 'send crap past the radar' (the radar being conservative types, I imagine) by trying to say "well they're not really women, kinda maybe".

And there's no Asari companions in ME: 2 so all you can do in that game is continue a Liara romance.