Reeba, I read it as you do - that he wants Marianne to become capable of falling in love a second time (with him!), but not at the cost of her idealism and innocence.

Maybe he's afraid that Willougby's departure will leave M thinking that "love" is no more than an illusion or a social game, and that no lover is sincere or worthy of trust -- these would be the "too common, and too dangerous" opinions expressed in many novels of the period (I'm thinking of "Dangerous Liaisons," for example... not that the Dashwoods would have been reading THAT! but along those lines.)