Adults only cruise?
I don't think RCI offers an "adults only" cruise. When we were on the Sovereign for a 4-day last year, it was far from adults only.
In fact, if the OP sails during the summer, there will be a number of families with children. But that's true of every cruise ship in the summer months. The 4-day to the Bahamas is a pleasant (but too brief) cruise. RCI food is good, entertainment is fun, with one "headliner" night and a second show night along with the two production shows by RCI singers/dancers. Service is superb. Cost is usually half of what we found for Disney. GrayLine runs shuttles frequently from MCO airport to Port Canaveral. Embarkation is smooth. Here is link for other transportation options: http://www.portcanaveral.org/portinfo/groundtrans.htm
Enjoy your cruise. You could probably take two trips for the price of one Disney cruise.