When my mom asked me (I'm Steffanie, seventeen years old) to deliver my fathers forgotten lunch to him on my way to a friend of mine, I said of course. Entering the building is done normally by reception at the front, but on Saturdays the building is largely empty. I know my father leaves the service door open at the side of the building as there's no receptionist on Saturdays, so that's where I went in. His office is two floors up, top floor at the rear of the building. The internal blinds of his office were closed which was unusual and when I reached his office door it was locked but I could hear someone talking inside. I moved around a little to a gap in the blinds and saw a naked man stood up in front of my fathers desk, his ass moving in and out. Squatting down some to get a better view I looked again, this time I could see my father lay spread face down across his own desk with his trousers around his ankles. Behind him was a young guy who'd not long started with the firm, he was fucking my fathers ass and my father was asking him to fuck him harder. It was my fathers voice I'd heard at the door. Not wanting to wait around and totally shocked I took myself and my fathers lunch to my friends house where I very nearly phoned my mom. I didn't and when I got home much later I didn't say anything to my father or mother what I'd seen and heard. It's been over a month since I saw my father being fucked and each time I go to his office, which is normally twice a week I can't get the vision of him bending over his desk with his employee's cock up his ass. It's becoming increasingly hard not to say something because I know every Thursday or Friday he asks the young guy to do some work on the Saturday with him. It's obvious to me now he's having a gay sex affair with him when their alone each week, and it's becoming a weekly event. My father doesn't have any notion that I know about him and I'm wondering if I should let him know I know, before things go any further...