The standard should not be "Don't do anything you wouldn't do in front of your boss," but rather "Don't do anything you wouldn't do in front of the neighbors' kids." The airplane cabin is NOT a workplace populated only by adults (as some people too often fail to recognize).

It's a pretty immature adult who cannot suspend his/her need for R- or X-rated entertainment for the short period of an airline flight.

This applies not just to sexual material, but also to other things. I wouldn't watch violent DVDs on my computer player for same reason -- because it would be unfair of me to accidentally expose some parent's 6-year-old to that material. Taking a kid on a plane should not be an opportunity for putting them at risk for this kind of stuff.