In FMA with all the Homunculi, their clothing appears to be a part of them. They can put on more, but their base form has clothing as part of it (minus Envy and we can presume Pride). With their clothing, they appear to have little "nodes" on them; with Greed in particular his nodes are on his sides and shoulders. Lust has them on her arms.
Now this might be a tad NSFW, as I have been told those markings, the lines with the nodes, have appeared in other things--to be particular, homosexual adult films. I spoke with a friend of mine about this, and he said it was a popular tattoo in the BDSM world. I can find zero proof of this. I have only seen one person with them as tattoos, his name is Shawn Morales; he's appeared on TV, and it seems he didn't even know they were from an anime.
So would anyone know the origin/meaning of the markings on a few of the Homunculi? The bondage connection makes sense as they're to a degree bound to father, but I was wondering if there was anything more to this.