Well... I was on a major Asian carrier last week a couple of rows back from the front, and they certainly had a (m& f) couple in civvies in the jumpseats for most of the flight. Well actually, mainly for the t/o and landing, because being an A340 on a 4 hour run from a well know honeymoon destination they then apparently were allowed to take advantage of the crew rest facility behind the cockpit. I can't imagine what for
Now, the guy looked like he might have been a colleague (he had the almost-crew-cut and certain air of superiority that crew have when on board) but I would be very surprised if the girl was, although I did take my first flight with this carrier with a female F/O the week before.
So I guess it depends who you know...