Honestly, I'm not deep into the 'Doc Martin' fandom but I've browsed enough to see patterns. A lot of the M-rated stuff zeroes in on the unresolved, almost painful tension between Martin and Louisa. Writers take that canon frustration and just... crank it up. You get a lot of 'what if' scenarios about their wedding night going wrong in a different, more intimate way, or explorations of their first time being awkward and fraught with miscommunication in a very adult sense. It's less about pure smut and more about the emotional rawness of two stubborn people finally being physically vulnerable.
There's also a surprising amount of darker, psychological stuff tagged M. Martin's past trauma and his possible neurodivergence get explored in really intense ways, sometimes with Louisa as a support but often just him grappling with his own mind. I've seen a few where he has nightmares so severe it affects their marriage, or where his clinical detachment bleeds into their private life in a damaging way. It's heavy, but feels true to the characters. The Cornish setting often becomes this isolating, almost gothic backdrop in those stories.
You'll also find the occasional AU that shifts the rating, like a modern city meet-cute that still carries their fundamental personality clashes into more explicit dating scenarios. Those can feel a bit out of left field for the show's tone, but they're out there. The appeal seems to be taking the core dynamic and placing it under a high-stress, adult-pressure magnifying glass.