Personally I think tattoos are sexy, I love a man with a little (or a lot) of ink. Adding a tattoo or scars to character adds dimension and adds to that bad boy persona us romance addicts seem to crave.
J.R. Ward put tattoos on her vamps in the Black Dagger Brotherhood (not all m/m but hints at gay relationships) and in the Midnight Breed (not a m/m but is paranormal) by Lara Adrian has the male vamps with color changing dermaglyph markings as a sign of a linage while the female breedmates (would be even hotter if one of the breedmates was a male) have special crescent moon/teardrop tattoos.
I also think that scars are sexy and very unique in stories. Another example again is in In Viki Lyn’s (historical/paranormal m/m), her character Jack has scars from working for the Sentinel and surviving a torture session that left him supernatural. Scars show survival and a man with a little damage is tougher to tame, so when they fall in love they fall hard, which I always enjoy reading. In Stephani Hecht’s Drone Vampire Chronicles (m/m, m/f, menage) the vampires bond with their mates by biting them and leaving a scar so others know they are taken, so sexy.