I don't think it is a sexual activity watching 'adult' web sites.
If watching porn isn't a sexual activity, I don't know what is. Likely there's some kind of wanksterbation going on, because people generally don't watch porn for any other purpose.
Hopefully people won't fall for this shit, but unfortunately I think there's always a gullible population that will. No matter how insane scams like these sound, they play on the fear of being embarrassed and that can be powerful when combined with ignorance. I didn't get this e-mail, but props to OP for posting this.
I admit my heart did sink for a few seconds then I realized obviously there is nothing to worry about.
Yeah, and that's exactly the reaction the scammer is looking for. Luckily you're not gullible.