Love is a driving force in Water for Elephants, and it's not limited to the love one human feels for another. People also love animals, and animals love them back. It shouldn't be a surprise that in the prologue we can't tell whether Jacob is talking about Marlena or Rosie when he uses the word "she" – he loves them both and thinks of them both as individuals.
Human love can be found inside or outside of marriage in this novel; the two don't necessarily go together. Jacob and Marlena fall in love while she's bound to someone else, and they make their emotional and physical pledges to one another in spite of that. Do you think the author, Sara Gruen, makes a judgment call on this? Or does she let the readers decide for themselves?
In both cases, the love we see in Water for Elephants is far from traditional.