I want to push back slightly against the idea that's an "extended sex scene", or even that it's a "sex scene" at all.
It's one of the most beautiful sequences in all of cinema -- a grieving couple rediscovering intimacy and joy. The way it is intercut with them dressing, getting ready for dinner, the way you can see it brought happiness and affirmation and some sense that life is not over and love survived a terrible loss. It's central to the entire film: to why they are in Venice in the first place, to their commitment to each other, to their determined love for each other.