There is something about an older man with a younger woman, something iconic or if you will fetishistic. One of the truly vivid examples of that in fanfiction is the startling number of people who seem to see Hermione and Snape as a couple in Harry Potter (which includes a very well-done and hilarious music video on YouTube called "Buttons"). Along the same lines, I recently watched the movie For Your Eyes Only wherein Holly Johnson's character had a huge crush on Roger Moore's James Bond (there was a three-decade difference in their ages, I believe). Then look at Monica and Richard on Friends.
Having said that, I do believe in this case it is evidence of Carolyn's issues. Boy has she got some! [spoiler]Methinks maybe that helps explain why she behaved so differently from either Maggie or Willie when bitten by Barnabas. On some level she rather enjoyed being subject to a mysterious, dashing older man. This was one thing I also thought worked well in HODS, her intense interest/jealousy of Barnabas. And it certainly goes a long way to explaining some of her bizarre attraction to her husband.[/spoiler]