The 28-year-old man and 48-year-old woman were apparently strangers before boarding the flight.
Both the man and woman were traveling to Detroit to catch separate connecting flights: He was going on to Miami, and she to Nashville.
Two Delta Air Lines passengers were arrested and issued citations on Sunday after one was discovered performing oral sex on the other in their seats.
Officials say they had met only a short while earlier on the plane.
A fellow passenger said: "The act itself is inappropriate in a public space". "There are families. There are seniors". Just two complete strangers getting it on in the middle of their row on a moving airplane.More news: Warm relations as Turnbull and Ardern stow differences in chilly bin
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FBI agents said the incident is still under investigation, and the two could be charged with anything from a misdemeanor to a felony. "These things should be respected".
The couple who engaged in a sex act on a recent Delta flight isn't likely to face charges for their lewd behavior, according to a former federal prosecutor.
Once they landed in Detroit, the man and woman were issued citations and are now facing criminal charges from the Federal Bureau of Investigation.
"If people will go so far as to disrespect themselves and disrespect you, something should be done", a traveler said. Both were issued citations.