A bad one that makes no sense to begin with due to the aforementioned fundamental differences between the subjects, even leaving the obvious offensive potential aside. Might as well have been trying to make some kind of point about breathing by comparing it to making sandcastles.
Breathing is like making sandcastles. We learn it as kids and it stays with us for the rest of our lives.
There, what's the problem? The point of analogies is to isolate certain characteristics you then make a statement of. When you compare space to rubber you compare something that's utterly different and incomparable, but rubber's flexible, and so's space, and that's what you want to talk about.
Of course on closer examination you'll quickly realize that rubber is not, in fact, in any way like space. But that wasn't the point; the point was to illustrate a specific statement. Nothing more.