The Neighbour and the Voyeur

I was house sitting at my sister-in -law's place, a typical, narrow dutch apartment on a narrow street. I was watering the plants, feeding the rabbits, doing a bit of cleaning. Suddenly, I noticed that the  owner of the apartment across the street was sunbathing nude on her tiny roof terrace. Ashamed, I turned away and minded my own business. But sometime later I noticed a man join her and they were making out, unaware of my presence across the street. They were obviously enjoying themselves in the hot sun and I couldn't help getting a hard-on instantly. I jerked off frantically, my eyes glued to that terrace across the street, where these two people shamelessly had fun in the open air.