I'm pretty sure that most of the solid anti-war protestors would expect Ukraine to just accept Russian territorial demands, up to and including complete annexation.

It turns into a reductio ad absurdum pretty quickly though. Putin didn't seem to return Crimea or the occupied regions of Donbass and Luhansk when asked politely. Not even when asked sternly. Indeed, it would seem that when all he faced was stern disapproval he decided to come back for more.

There is no doubt in my mind that supporting Ukraine now is stopping more Russian aggression later. Besides, Putin can end this war any time. Just go back to the original borders. The only reason not to is his yearning for Imperial glory. The irony being that many of these anti-war protestors would probably proclaim themselves anti-colonialists.