I see bite marks very similar to those on our females, from time to time. As long as your water is clean and they get fed, they heal like magic. I highly doubt that the sand is a factor in that.
We just put our largest female motoro(xmantilla?) into our 600 gallon tank with our Leopoldi/Pearl male. They are mature, and I'm hoping to see more of those bite marks in the future! Good sign, as long as it doesn't get out of hand.
If it gets particularly gruesome, you can separate them for a few weeks until it heals up. From what I gathered from Mike at H20Custom, sometimes certain males can get over-aggressive and need a "time out", of sorts.