Anal penetration. This includes anal sex, anal fingering, anal insertion of sex toys, anal tentacle penetration, and any other type of anal penetration.
If, and only if, a penis is inserted there, then the image should also be tagged sex.
The following tags are aliased to this tag: anal_penetration and anal_sex (learn more).
The following tags implicate this tag: anal_fisting, anal_object_insertion, and multiple_anal (learn more).