I tried to explain this before, but didn't do a very good job. I'll try again. The vagina itself is like an uninflated balloon, and the reason you feel friction when a penis is inserted is due to the muscle tissue that surrounds the vagina, not the vagina itself. Having a baby vaginally (or I suppose inserting something kind of equivelent in size) are the only real ways to damage the muscle tissue enough to make one "loose". (I don't know about the post-partum "loosness" from personal experience as I have only given birth through C-section, but this is what my OB-GYN told me.)