My dear Explicator, I suspect that you were not a normal kid. It would explain much. :^)
The speculation that Joe and Marian did not engage in conjugal relations because Bob was the only living child is frankly a little weird. You have no idea if there were other siblings that died or were still born. Maybe Marian couldn't carry to term. Maybe Joe wore his jeans too tight. Maybe Bob was adopted. Maybe something went wrong after Bob was born and Marian couldn't conceive again. Maybe they practiced birth control. (Yes they did have it back then. If you are interested in how Freep mail me and I will tell you some of the more common methods) You don't know because Bob didn't say but that would not have been the kind of thing he would talk about in any case.
But to jump from them having only one living child at the time of the story to Joe and Marian having a celibate marriage is worthy of an Olympic gold medal.
a.cricket