For what reason does the wife keep asking Robert if he’d like to go to bed in "The Cathedral"? What effect does Robert’s reply have on the...

The most straightforward reason why she keeps asking is that she herself is tired and wants to go to bed, but can't until her guest goes to bed.  If you have ever had guests over, and wanted to go to bed, a good way to hint around is to hint politely, as she did, "Your bed is made up when you feel like going to bed, Robert. I know you must have had a long day. When you’re ready to go to bed, say so."  But, he doesn't go to bed and neither does her husband, so she eventually just dozes off right there on the couch.  The story really doesn't indicate that there is any reaction from the husband whatsoever.  She asks Robert twice to tell them when he was ready to go to sleep, and neither time does the narrator react. He has other comments throughout though.  For example, when she goes upstairs to change and takes a long time he states, "I wished she’d come back downstairs. I didn’t want to be left alone with a blind man".  Then, when she falls asleep, he states, "I wish my wife hadn't pooped out."  So initially he is resentful that he has to be there with Robert, alone, playing host.  It is uncomfortable for him.  He does it though, and in the end, has a really neat experience drawing the cathedral, so it wasn't too bad after all.