![1boy 1girl ahoge animal_ear_fluff animal_ears anus between_breasts blush bound bound_wrists breasts breasts_apart censored eyebrows_visible_through_hair grey_hair hair_between_eyes hasu_(hk_works) hetero imminent_sex imminent_vaginal invisible_penis kneepits large_breasts looking_at_viewer lying mosaic_censoring necktie necktie_between_breasts nipples nude on_back original pussy raised_eyebrows red_eyes red_neckwear shiny shiny_skin short_hair solo_focus spread_legs sweat tail thighhighs v_legs white_legwear rating:e score:23 post #3615505](/cdn_image/preview/c2/17/c2177f2852adfd9459b43efbe9df82ad.jpg)
When a man's penis or a foreign object is about to be inserted into a female partner's vagina. This is the step before the vaginal occurs. Whether or not this is occurring in an imminent rape (against the woman's volition) or imminent sex (consensual sex) is not important here.
This is different from grinding. There, the female's pussy is rubbing against the penis. Here, the penis is actually being guided into the vagina. The key visual difference is whether or not the penis' foreskin is positioned just outside the vaginal cavity.
Also, try not to confuse this with after vaginal, 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.
Likewise, it can count as sex if the female partner is already in the middle of anal sex with one male partner, while a second male is about to put his penis in her vagina. See imminent double penetration for further details.