In summers, it's often very hot in Madrid. On such a hot summers day, t's really nice to hang around the Lago, a swimming pool outside the city (get out a the metro station with the same name). This is the place the locals get a refreshing swim.
There are usually lots of gay guys at the top swimming pool. In hot weekends Lago is usually very popular; sometimes you have to wait in the queue for almost an hour!
Go left (stairs up) after you entered the area. Stairs up to the very top. After you have reached the top you have to go straight ahead to the other side of the pool. There is the place where all the gays and friends are.