Going back to the OP we have 'express intense pleasure and triumph' coupled with 'euphemism for the sex act performed illicitly'. And we have specifically 'gesture of the fingers' (not a fist). To me a gesture of the fingers suggests perhaps moving the fingers as opposed to just holding them static. Intriguing and further evidence needed. The V sign does usually involve either wagging the fingers or an upward thrust as in the 1901 video. There is an obvious connection between 'triumph' and 'victory'. To a soldier could this also signify 'intense pleasure'?