Khloe K & James Harden -- The Couple That Spins Together ...

When a couple WORKS OUT together -- you know it's serious.

Which is why it was interesting that NBA star James Harden took Khloe Kardashian to a spin class in Hollywood last night ... proving their weekend in Vegas was no fling.

The two hit up Cycle House -- where Harden has hit the bike in the past ... and also happens to be a place featured on an E! reality show called, "Hollywood Cycle."

And with "Keeping up with the Kardashians" also on E! -- seems like the perfect date night, combined with the perfect amount of promotion ... just how the Kardashians like it.