Passionate public kissing and social stability

As I walked quietly into my darkened house one evening after a day of work, I was startled to see an alien creature prostrate on the sofa. It appeared to have a single body with two heads engaged in an activity that was hard to make out.

A sort of wet slurping sound was audible - as if the creature were feasting on human flesh.

I flipped on the lights, and the alien uncoiled abruptly, as if caught in the act of whatever aliens do.