I was taking a look at the work of HR Giger and came across an unreleased film he was helping to create. The film was going to be about aliens who were disguised as humans and taking part in everyday life. Within the more detailed description I came across this quote:

"Anxiety kills these beings, while sex can either be a savior that rescues them from morphological breakdown, or it can kill both the alien and the chosen human mate in a life-draining cocoon."

http://www.hrgiger.com/frame.htm

Under Film and Theatre "The Tourist"