In a second photo on the post, the unlikely foursome huddled together in the hot tub for a selfie.
The celebrities added more fuel to the fire, with Beckham commenting:
"I mean come on it’s Monica, Mich & Cam 💗 can it get any better ?"
Eric Stonestreet commented, "I liked it," while Jesse Tyler hilariously wrote, "Sorry I was so gassy."
A shocked Jennifer Aniston - who is rumoured to be romancing her former onscreen lover, David Schwimmer - echoed everyone else's thoughts when she commented: "Wait....WHAT'S HAPPENING?!!?? 😳"
Fashion designer, Rachel Zoe concurred, commenting "Seriously what’s happening here?" beneath the provocative photo.
However, there is a rational explanation for the shocking photos.
According to Entertainment Weekly, Cox and Beckham will appear in an episode of the hit comedy series Modern Family in 2020.
The mega-stars will play themselves as they compete together in a celebrity bowling tournament.
Cox and Beckham will find themselves entangled in a family drama involving "’80s aerobics classes, a terrifying pool slide, and three side-by-side hot tubs."
Cox is currently in a relationship with Snow Patrol's Johnny McDaid, with whom she was previously engaged to.
"Isn't that strange? We broke off our engagement and he moved to England, then we got back together, and it's actually better than it was before. Everything is better," she said.
"Not because he's in London, although the distance after that breakup - we were apart for six months - that really showed us a lot, and it's just better."