This tag is for consensual vaginal or anal sex only. Don't use it for other sex acts. Add imminent vaginal and/or imminent anal as well to show which activity is about to take place. If it is non-consensual sexual activity that is about to occur, use imminent rape instead.
Don't use this tag if you don't see penetration about to occur. Images like post #865022 or post #1570172 shouldn't have this tag because it is circumstantially implied, but hasn't been demonstrated to happen yet. The easiest guideline to follow is, "If you don't see the penis about to enter the cavity, don't use this tag."