Edit
Two lovers who are about to engage in consensual sexual activity, but have not actually penetrated each other yet.
Both anal and vaginal have the definition, "If, and only if, a penis is inserted there, then the image should also be tagged sex." Since this tag depicts the moments right before sex, it should also reflect the fact that only a penis that is about to be inserted into the vagina and/or anus should have this tag.
Consider adding imminent vaginal and/or imminent anal as well to show which orifice is about to be penetrated. This only covers the general fact that sex is about to occur.
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 a penis about to be inserted into the anus or vagina, or the receiving partner is obviously positioned to allow for intercourse 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."