You could check out the various singles cruises. (just google it). Although many are for more of a 35-60 crowd, there are ALWAYS some in their 20's. Also some of sites actually specify certain age groups.
Have fun. I have a feeling if you guys just go on any cruise and visit the bars and disco you won't have any problem. You probably also want to select late dining.
p.s. Carnival is also probably your best choice for cruiseline.
Empress of the Seas 4/21/06
Star Princess Singles Cruise 10/30/2005
Carnival Valor Singles Cruise 1/16/05
RCCL Mariner of the Seas 9/12/2004
Spirit of Chicago 2-hr Cruise 7/24/04
RCCL Monarch of the Seas 3/29/04
Costa Mediterranea 12/14/03
RCCL Sovereign of the Seas 9/21/03