You laugh, but you have no idea how badly I messed this up…
Oh no… They're in trouble also. I messed up so badly, and they're not to blame either.
Not yet…
Well, the young couple had sex. You know what this means—they are doomed.
Shows where lots of people die tend to have a strange conservatism about who gets killed. If anyone engages in nonmarital sex, especially unprotected and/or with someone they don't really know, you should probably consider them to have a crosshair on their back, even if the killer is choosing their victims totally at random. Fanservice Extras are particularly vulnerable to this trope.
Very common in slasher movies, such as the Friday the 13th series, and the entire basis of such Dark Fantasy and Supernatural Thriller films as Liquid Sky and It Follows. This could be a metaphor for the AIDS scare (then-new in the case of older films), or for STDs in general, although according to one of the makers of A Nightmare on Elm Street, it was simply because he thought that people having sex will forget about everything else and be especially vulnerable to serial killers. Which wouldn't be an Ass Pull if they only died during sex, but when they're prone to it afterward, it is hard not to interpret it as anything but a cautionary message.