My recent entry on berries reminded me of an email I received from a friend, quite a while back, following a conversation where we debated the origin of "playing gooseberry":

I did a quick Google search on "playing gooseberry" (as in being the "third wheel") and came across the following link, which suggests that "gooseberry" is a euphemism for the devil. What do you think?

Why not? "Gooseberry" is indeed one of the many names given to the devil, and there is no doubt that he would be an undesirable third party in any tryst. However, the Bloomsbury dictionary of word origin suggests another origin:

"play gooseberry", or to be an uncomfortably superfluous 3rd person between two lovers, goes back to the early 19th century and may have originated in the notion of a chaperone (ostensibly) occupying herself with picking gooseberries while the couple being chaperoned did what they were doing (gooseberry-picker was an early 19th century term for a chaperone).

The Oxford English Dictionary confirms this use of "gooseberry-picker", so this explanation looks quite convincing.
In French, the equivalent expression is « tenir la chandelle » (to hold the candle). The Dictionnaire d’expressions et locutions tells us that one custom around the wedding night used to involve a best man, who had to provide light for the newlyweds. He would turn his back on them while holding a candle.
I think I’d rather be a gooseberry-picker than a candle-holder.