Fark! If someone other than my DH had the audacity to motorboat me, even in jest, I would punch them in the face. End of.

There is what I call 'friendly banter' and then there is flirting. I think if it's being done in secret, there is more to it than just flirting. People can have an emotional affair without even touching someone else or even being in the same room. IMO it is not OK.
Don't get me wrong, I realise I am definitely not 'the norm' there. Boob grabbing seems to be the thing with my friends and I so I don't even bat an eyelid when it happens. I am VERY close with those friends though, if it was some random it would be a different story. I should also add that he didn't just come up and do it, I did say it was ok before he did.

You're right, there's friendly banter and there is flirting. So I guess what is and isn't ok would depend on where you think that line is.