I know exactly why I couldn't do DudeShep/Liara, and it's two reasons:
1) My one and only DudeShep playthrough, I ran around being a total jerk to everyone. I just couldn't imagine Spacer/WarHero ShepLoo not being a full-of-himself self-important a-hole. And then, I couldn't imagine Liara being truly interested in someone like that, so it didn't work for me. Maybe if I'd tried a DudeShep run with a custom face and less "Yes, I am God's Gift To Ass-Kicking" background, I could have made that work.
2) I'm not going to say that one VA is better than another, though I prefer Hale - but between the different look and the different VA, FemShep and DudeShep feel like two completely different characters, even though they're doing the same things and delivering the same lines. Different FemSheps still feel like variations on FemShep - DudeShep felt like something else entirely. After playing through many times as FemShep, Liara felt like FemShep's love interest. When Liara expressed interest in DudeShep, who - again - felt like an entirely different character even though they're both Commander Shepard, it was weirdly like she was cheating. "But ... Liara ... you belong with this other character! Who ... doesn't exist in this playthrough. But, still - stop that! I'll play FemShep next, I promise!"
So, to sum up - my ShepLoo was a jerk, and my FemShep saw her first, so she had dibs.