A reminder to engage in safe sex. Usually used when the people in question aren't actually planning on having sex, but the speaker is implying that they will be having sex.
Person A is leaving the party with person B, a platonic friend.
When a man will search for hours to find something that is laying out in the open on a table. Items are often easily found by a women.
Man: "I have been searching for hours for keys."
Woman: "You mean the ones sitting there on the coffee table?"
Man: "Where?"
Woman: "Right there in the middle of that table."
Man: "oh, must have been Male Pattern Blindness"