Walk In!?

What do you do if your older child walks in on you and hubby/girlfriend/significant other while y'all are doing the dirty?? My hubby and I decided to get a little frisky on the couch and our 4 yr old came out and asked for milk! I'm not sure how long he was standing there, as he was behind me while I was cowgirl, but he was half asleep and actually went back to bed and passed out before either of us could even get up! I'm curious as to what you do/would do in that situation