The movie is from the 2000s or maybe 90s? Likely a danish, swedish or scandinavian film. There is one sex scene inside a house, close to a window. Two teenage boys are watching from outside the window, masturbating. After this scene the boys are swimming in a lake close to the house.
my immediate thought is one of the america pie movies, but they are not those nationality, so look further i must!! (Et language)