You adjust the straps so that the harness is actually around your stomach as opposed to where the girls would normally wear it, across the pubic area. I have seen it done once in a porn mag so I know it can be done. The reason that the thing is hollow would probably be an idea so that it's soft and more appealing to when girls use it on other girls. Or when a girl ass fuck her man and because it's hollow actually squeezes together to fit into the tighter region.

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An erotic story
The instructor and student
Secretary learns how to take a dicktation
The story of sister stephanie