A simple vanilla error, one that has managed to be preserved in Aric's mod and slip by despite having been found a long time ago. It's actually the skin cover that is causing it, not the multple nipples. Line 1698 in sexdescriptions.gd is:
return getRandStr(descTitsBodyType[3])
but should be:
return getRandStr(descTitsBodyType[1])
The index 3 should be 1.