A gelatinous Cube and a 10-feet corridor ...

[Umbran]

But the fighter isn't pushed; the Cube WANTS him to stay in its space - the Fighter pushes HIMSELF away from the Cube - ergo, it's a move he's doing willingly.

But it isn't his move. His move only happens on his own turn, and it is the cube's turn.

 

[77IM]

Thanks for all the suggestions, guys!

So basically, it's yet another arbitrary situation where I have to be the final umpire and decide whether I want to punish the PCs for sticking together, or throw them a bone and let a successful save be a successful save.

*sigh* decisions, decisions ...

Don't think of it this way! If you find yourself thinking in these terms ("punish PCs" vs. "throw them a bone") here are two easy outs:

1) What would be the more interesting outcome? "Fun" can be hard to pin down but "interesting" is usually easier to recognize. Forcing the players to make a decision is usually interesting. Presenting the players with a mystery, tempting unknown (the Big Red Button), or twist, is also usually interesting, but it's probably not applicable to this cube situation.

2) Let the dice decide! Have the fighter make an Athletics check, and on a success, he winds up somewhere helpful -- and on a failure, he's cube food. This way, the "logical but punishing" option is on the table, but the player still has a chance to get what they want.

 

[Swarmkeeper]

But the fighter isn't pushed; the Cube WANTS him to stay in its space - the Fighter pushes HIMSELF away from the Cube - ergo, it's a move he's doing willingly.

Except for exactly what the GC stat block says:

“On a successful save, the creature can choose to be pushed 5 feet back or to the side of the cube. A creature that chooses not to be pushed suffers the consequences of a failed saving throw.”

Being pushed is not a move (nor is it something you do to yourself). Or are you telling players at your table that they have 5 less feet to move after getting pushed?

 

[Rya.Reisender]

I don't consider being pushed back or being forced to move back to avoid being disintegrated as willing movement.

 

[jasper]

Be pushed is not a willing movement it is forced movement, the monster is being neighborly by letting lunch choose. Who ever else next needs to spend 5 feet of movement to leave the square and back out of the way.
Now as an EVIL DM. I would drop 10 by 10 block of stone 5 feet behind the wizard. Then everyone is getting jelly!

 

[guachi]

I've had this situation actually happen. The PC got engulfed because she had nowhere to go. They defeated the gelatinous cube. She got her gelatinous cube sample she desperately wanted (it was a Ranger with "Oozes" as her favored enemy. We had more fun than is legal with that choice).

No complaints. The party moved on and learned a minor lesson in better tactical choices because so often it doesn't matter.