The reality vet just launched her first scent, "Hard Feelings," from her Uncommon James lifestyle brand -- and the ad is hotter than the desert sun ... with Mark featured in the quick spot, getting all touchy-feely with her amid the cacti.

Check it out -- Kristin stumbles upon him out in the desert, who promptly sweeps her off her feet ... literally. Other smoldering shots show KC giggling as he wraps her up from behind.

Naturally, there are plenty of sultry glimpses of the fragrance itself, which is meant to be unisex -- so it makes total sense why Mark’s making an appearance in the ad ... Kristin is trying to advertise this to guys and gals alike.

But let’s be real -- Mark's role in the fragrance campaign is a clear sign of how serious their relationship is ... especially with Kristin trusting him enough to put him front and center in a major project for her company. So yeah, they're knee deep as a couple now.

As we've reported ... KC and ME kicked off their relationship in February, and it's been blazing hot ever since, and they’ve even chatted about future baby plans.