imminent anal
When a man's penis or a foreign object is about to be inserted in the receiving partner's anus, but hasn't actually penetrated yet. This is the step right before anal occurs. Whether or not it is imminent rape (against the recipient's volition) or imminent sex (consensual) is not important here.
Try not to confuse this with after anal, which may look similar to this in small thumbnails.
While this is not considered sex quite yet, it is possible to have both tags if both the intended penetration and the actual penetration are depicted in the same image, typically in a comic.
Examples
Likewise, it can count as sex if a female partner is already in the middle of vaginal sex with one male partner, while a second male is about to put his penis in her anus. See imminent double penetration for further details.