The Harkness Test is a meme image describing what you can ethically romance in fiction, be it in reference to video games, roleplaying games, fiction, fanfiction, or what have you. Not that this will stop the particularly depraved, and to some it's more of a statement of what is dull.

The meme originates from a character named Captain Jack Harkness that first appeared on Doctor Who, later on as an important character on the spinoff Torchwood, before returning to Doctor Who. The character is a flirtatious pansexual action hero who regularly deals with exotic aliens and humans alike.

Also note that it doesn't hold much weight with professional psychologists, who would regard the attraction to animal junk to be zoophilia regardless of whether the creature attached to them can talk.