It's not so much a matter of "clinging" to the stance that having all of the female characters be romanceable DOES send that message, but a concern that the decision to make all the females in the main cast romanceable comes out of or reinforces the already existing perceptions of women as universally available sexual objects. It's something I had noticed as well and initially assumed that it would mean that Vivienne was out of the running after the other 3 women were all confirmed. I eventually decided Bioware could probably still make Vivienne romanceable without it being problematic (even, as TheGus pointed out, that it might be problematic if she's NOT), given that their romances don't treat women as objects and that they decided not to make Morrigan or Leliana romanceable.
But, as Judas said, the status quo in games tends to be that female NPCs are winnable objects for male PCs, so it's still a concern worth voicing and most likely something the devs took into account when planning the romances.
I've wondered about this stance too.
Here's the thing. Sera is available to females only, Cass is available to males only, so you have, in any given playthrough, at least one female who is not available to that inquisitor. This would appear to resolve your concerns, no? As for females being 'winnable objects' in games.. well if that's the case, then so are the male romance options. And in a way they are. They are not real people. They were added to the game to be romance options, in part. Really, they were added to the game for the player to interact with in a variety if ways.
I think its clear there aren't enough female companions, and the devs rationale behind this is just bizarre, so yes as someone else said, Bioware has painted themselves into a corner.
In any case, even ignoring these factors, this will be the first DA game where this is the case. Aveline and Wynne were not romancable at all.
The thing that really boggles my mind is that people think this has anything at all to do with real life. No-one, not one single person, regards every female they meet as a potential sex partner. (well, unless they are off their rocker) No-one is going to start thinking that because of a video game.