I don't know if Linda was talking about fantasising or about the real thing but I'm not fussed either way. What consenting adults choose to do just doesn't affect me, although it's not something I'd be into.
There was a film called The Ice Storm set in the 70s with Sigourney Weaver and Elijah Wood that had a 'throw your keys in a bowl' suburban swingers party at its centre. I asked an older person if this went on a lot, the whole pampas grass outside to signal you were a swinger, and they confirmed it did.
If swinging was/is big in suburbia, it's undoubtedly happening in showbiz circles. And I know a load of people, ostensibly 'normal', who've at least had threesomes.