I'm obviously not complaining...
But, if I disregard my sick, twisted fangirl enjoyment of Billie Joe making an ass of himself, I still don't think I would ever have a problem with him groping himself, making sex noises, etc. I mean, it's kind of like his response to the angry parent that inspired the song Reject — he's not going to dumb down his music for the eight-year-old kids that might be in the audience. A lot of the band's lyrical content and themes are just as offensive as a grown man putting his hand down his pants and moaning in jest. Any parent who is stupid enough to take their young child to a concert before researching the band or performer doesn't have the right to be offended, tbh. So often parents place the blame on others instead of taking responsibility for their own inability to properly parent. The reason I didn't get to see Green Day on the American Idiot Tour is because my parents backtracked on their plan to buy me tickets after they decided the band wasn't "appropriate" for a 14-year-old. Aaaand that's why I am the way I am today.
But, anyway... it's basically just a "shock value" stunt. I doubt he actually finds himself sexy while doing it. I assume it's just meant to be funny and idiotic, not offensive, serious or actually sexy. (Even though the man looks good doing basically anything...)