It's not that it's abrasive, though some soaps do have abrasives in them for "exfoliation" and that is also bad for the dick. The problem is that it strips all the oils from your skin, and oil is what keeps skin stretchy. You fuck a bar of soap (or use soap as lube, whatever) it'll strip all the oils off your genitals and then the minute the skin goes to stretch (like if you're fucking, or masturbating, or get aroused) - RRRRIPPPP, microtears and friction burns everywhere.