after sex
After the act of intercourse is finished. Telltale signs include used tissues and used condoms, cum dripping out of any particular orifice, and heavy blushing.
Sometimes a character will light up a cigarette while the other cries. This occurs when a character is ruined for marriage.
If the persons involved are still emotionally focused on each other, and it seems that the sex still hasn't really finished, but is merely winding down, then afterglow should be used instead. Also note that, while they can coexist, these two tags tend to be mutually exclusive due to reasons of conflicting mood and style.
Try not to confuse this with imminent penetration, which are the moments immediately before sex occurs.