explain "marriage" as Arnoplhe's greatest desire and, at the same time, his greatest fear.

Just to let you know, this is under the wrong category. Arnolphe is like a fifty-two year old man. He has groomed Agnès since she was four years old (yes, this is beyond creepy) so he could have a young wife when she comes of age. This seems to be this guy's greatest desire. Keeping a young beautiful girl when he is a creepy old man is also Arnolphe's greatest fear.