My girlfriend and I were in the living room, entirely naked except for my girlfriend wearing a pair of flip flops. I was standing while my girlfriend was on her knees going down on me. Her sister entered the apartment and was quite surprised to see us naked in the living room with my penis in her sister's mouth. She stared at my penis, which is only about six inches, for several seconds before going into the bathroom so we could compose ourselves. She later told my girlfriend that it was the first penis she'd ever seen and was quite impressed.
My girlfriend had told her sister that we did everything except for sex, so you can imagine her surprise when she found us with my penis in my girlfriend's vagina. One day we were making love in my girlfriend's bed, with the door to the bedroom open. At some point the sister had come home without us hearing, and walked into the doorway leading to my girlfriend's bedroom. She must have been there for a little while before I saw her when my girlfriend shifted out of the way. Again she stared at my bare penis (we don't use condoms) for a little while before leaving.
I got really turned on by both of the encounters. Hope they interested you.