Oliver wants Delia to move in with him, but before she does she wants to know about his ex-wife Sylvia. As he describes her and their relationship (in flashbacks) Delia is astonished that he could have ever gone out with such a coarse and uncultured woman. He can’t explain it so Delia decides she needs to meet Sylvia for herself.
Inspired, I guess, by the many times in my life I’ve met the ex-boyfriends of women I went out with and had a similar reaction.