Ooh, what an interesting question! I would think the perinium is a different kind of 'skin' tho to our outside skin, and would stretch differently. I would think too that tearing was more positional than anything else.

I had patches of quite bad stretchmarks (I wasn't a roadmap that's for sure.. but my hips and on one inner thigh they were quite promininent) and I didn't tear.

Be interesting to see if there's an official answer to this one