I went back and listened, and you’re right, it’s mostly McCartney singing, with John joining in. But Lennon said he wrote that line about his relationships pre-Yoko. Why he stopped at Yoko, I’ll never know.
I still wonder how John got six bullets in the chest, and Chapman couldn’t bother to spare one for Yoko.
John contributed the "It can't get no worse" answer to Paul's "A little better, all the time".
As usual, the two did better with each other even if it was only something short like that or simply smiling.