In case you believe everyone is biased in favor of cruising regardless of the Norovirus, you'd probably be correct. But that doesn't mean they're wrong.
Previous posters have pointed out several things including Norovirus is the second most prevalent virus in the United States after the common cold and that Norovirus is studied by the CDC by using cruiseships simply because their contained environment make them perfect for the study of such a disease.
What however are your chances of getting sick? The answer is slim to the extreme.
Less than 2700 passengers aboard cruiseships last year were reported to have come down with gastrointestinal illness including Norovirus and many of these were in cruises in Europe. Yet over THIRTEEN MILLION passengers disembarked at American ports last year. In short, you had one chance in 48,149 last year of getting sick from a gastrointestinal illness on a cruise ending at a US Port.
Quite obviously you are an inexperienced cruiser as most cruisers are aware of how small the problem actually is. Therefore, should I be you, I certainly wouldn't let it bother me regardless of the ship I wished to sail.