Just came back from a cruise where a family group of about 20 people aged fro 3 to 80 wore tropical shirts with a pineapple motif.
This caused great hilarity from fellow cruisers.
"What does wearing a pineapple shirt mean? As pineapple is a tropical fruit, pineapple-print shirts are popular to wear on cruises. However, you should know that the pineapple is a secret code for swingers. By wearing pineapple clothing you may attract the attention of couples looking for a swinger party." So be aware.