http://arabic.al-khoei.org/falea.htm

Question 784: Can a Man touch the private parts of another man from behind the clothes and a woman touch those of another woman, jokingly and just for fun under the condition that it does not arouse Lust?
Answer: It’s not Haram if it does not (arouse Lust).
Source: Sirat al najah fi Ajwibat al istifta’at Part 3.