I think it's just funny, he's poking fun at the idea that he's this rockstar sex symbol, teasing the crowd by playing up to it for a moment before laughing at himself, and taking them out of their comfort zone at the same time (which he's always been an expert at!). Haven't seen him do the full wanking routine since AI times but he still does some teasing during Hitchin A Ride sometimes, humping his guitar and telling the crowd to make sex noises during the 21st CB tour and saying "do you want to touch it?" and stuff during the last tour , so he's still at it.