One designer has publicly stated that many years ago, yes. The situation could have changed since then, not to mention their cooperations with the Viera race's original creator may have yielded different results as well. It's fine to assume they'll be masculine, that's great, and it probably will happen. But to tell someone that it's impossible, with what we've been shown right now, for it to be a more feminine design is silly. You simply can't be certain without it being announced, let alone the fact that it's not absolutely certain male viera will even be playable at all.
Edit: Also let's not pretend that we've never seen the final product of a race differ VASTLY from it's concept art.
http://i.imgur.com/ipufbiY.jpg
