If you don't mind being last off the plane, get one of the seat pairs at the very back of the 777, where the layout becomes 2-4-2. Rows 46 to 48, seats A/B and J/K are good if they're available (row 46 feels quite roomy, but the armrest is immovable). You'll feel much less hemmed in here than if you were squished between your SO and a stranger.

If you want to get up and walk around / go to the bathroom frequently, I wouldn't recommend getting A/B or J/K seats in any other rows. Better to get D/E or F/G seats.

If your SO won't budge on getting a window seat, and the paired seats aren't available, I suggest getting an A/C or H/K pair close to the rear of the aircraft. The rear fills up last, so you may have an empty middle seat. If that seat is indeed taken and you want to sit next to your SO, I'm sure your neighbour will be more than happy to switch to the aisle seat.