I used to cut myself. I stopped when I started getting laid regularly. It's not that sex instantly changed my psyche so much, but it gave me nowhere to hide marks. My partners and I see each other fully nude with the lights on, and I wouldn't have it any other way. Saying "I was chopping vegetables" is dishonest and unconvincing; so is saying "yeah I cut myself, but just for fun, it doesn't mean anything."
Fortunately my skin heals well and even as an angsty teenager I was careful to never do real damage, so I don't have stripes on my arms or anything; the only self-inflicted marks on me are my pierced ears and the heart-shaped scar on my belly, and those aren't exactly mutilations.
Jon has an enormous scar across his chest. He was born with a serious birth defect and had emergency surgery. It's kind of funny to think that the incision must've been just a couple inches at the time, but the scar grew as he did. Now it's a broad slash across his ribs with a strange hollow dimple on one side. When we're lying in bed together I have a habit of running my finger along it and sticking my finger in the dimple. What's more intimate than feeling your friend's guts through his missing rib?
Brandon has a scar on one leg from an ATV accident--he severed an artery and needed a transfusion--and a scar on the other from having sex with me. (I didn't flip out and claw his skin off or anything, I just shoved him up against a rough wall too hard during sex and he didn't say anything until there was blood running down his leg.)
I'm at once embarrassed and proud that I, very literally, made a permanent mark on my lover.
I used to work with a guy who had his girlfriend's name tattooed in giant frilly letters on his neck. He was seventeen. It's possible to take the romance of permanent marks a little too far. At least when Brandon and I break up he can claim it's from the ATV accident or something.