No, because there are ways of not acting unilaterally that don't depend upon express consent.
Here's just one simple illustration, from 4e D&D play: a player builds their PC, and plays their PC, as a fanatical devotee of the Raven Queen; and so I, as GM, introduce undead, and Orcus cultists, as threats to the PC or to things the PC cares about.
That is not unilateral: the player is the one who has made the Raven Queen vs Orcus/Undead conflict salient. But it doesn't depend upon express consent.
Upthread, @chaochou talked about who authors the players' goals for their PCs. And I talked about the players exercising real influence over the significant content of the presented scenes, and their stakes, and what follows next. Those aren't exactly the same, but they're in the same general space. In my illustrative example, it is the player who - by way of their build and play of their PC - is shaping content and stakes.
To provide a fully-worked illustration, more would be needed. For instance, if the whole scenario is a GM-orchestrated "fetch quest" and the GM just replaces goblins with skeletons then, while the player has influenced the colour of the opposition, it is still the GM determining the stakes. And so I would still see that as pretty railroad-y. It becomes non-railroad-y when "threat to the PC or to things the PC cares about" is also responsive to player-established priorities. Which comes back to @chaochou's point about *who picks the PCs' goals?"
As I posted upthread,
There are plenty of ways of the GM adding to the fiction that don't involve the GM unilaterally deciding on the significant content of scenes, the stakes of scenes, and what comes next.
Correct me if I'm wrong, but wouldn't you, then, consider the traditional sandbox style (where the DM authors everything and sets all the stakes) as pretty railroady?