text: Depends how old the child is.  This happened to me when I was about fourteen.  There was no need to explain to me what they were doing, as I already knew.  If the child was younger, I would sit down with both parents. (this just reassures them) and explain to him that mom and dad need some time alone.  If you feel comfortable, it would be a good time to "have the talk" with them.  If not, just continue to explain as best you can....or like spiderman suggests, just avoid them.