![1girl 2boys anal aqua_(konosuba) bandana bar_censor blue_eyes blue_hair blush breasts censored colored_pubic_hair commentary_request green_pants hair_between_eyes hetero imminent_vaginal kono_subarashii_sekai_ni_shukufuku_wo! large_breasts long_hair md5_mismatch monchan_rev3 multiple_boys nipples open_mouth pants pants_pull penis pubic_hair pussy sex shirt_lift solo_focus spread_legs sweat testicles rating:e score:39 post #3586845](/cdn_image/preview/3a/06/3a0675ea9adce2071749d342554d24ce.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.