Stockholm has several documented outdoor cruising and nude-bathing spots, all seasonal and busiest on warm summer evenings: Langholmen (the island nude beach near Hornstull), Frescati (the woods and cliffs by Stockholm University) and Karson (near Drottningholm). Indoors, US Video runs a 24-hour cruise labyrinth in the city centre, and the Manhattan cinema on Kungsholmen is a men's cruise venue.
These are informal, tolerated areas rather than official venues. Be discreet, respect other visitors, and follow local law.