It's a pleasant fantasy but in real world, this situation usually goes awry, as others have mentioned. Maybe if a lot of alcohol or drugs are involved it takes the edge off the insecurities and performance issues that seem to arise inevitably. Also, some couples use group activity to spice up a relationship that's dying, and that's not a sexy situation to be in. It's almost impossible to arrange a threesome where the sexual chemistry is evenly distributed. In my experience, one person usually ends up becoming the audience or a kind of assistant.