A vagina can stretch to pass a whole baby... so if you are tearing her up you have to be mighty thick.
I've read a fair bit about this, and though it might seem logical, giving birth doesn't seem to relate to sex. Perhaps one difference is that the process of dilation at the end of pregnancy starts slowly over a period of weeks, takes several (painful) hours to complete, and is aided by a special hormonal process that prepares muscles and joints for stretching. I guess one way to look at it is to figure that Mother Nature (or evolution, if you rather) has designed the average vagina to do a pretty amazing job of tightly accomodating the average penis, and being able to consistently maintain that size in all situations except during delivery, and so that doesn't leave much room in most women for too much above average.
Vaginas don't seem to fundamentally stretch much during the short time frame of sex, though they do dilate a bit within limits. And sizes of vaginas seem to vary to begin with - and of course, those that have passed a baby will probably be a bit larger than they were originally. So a particular combination of relatively large penis and relatively small vagina may be too tight and cause problems. Anecdotally, I've seen some evidence that female partners may loosen up over time to accomodate their male partners, though some people also report that problems never really get enough better for their female partners. Of course, in the short term there are some factors of excitement, position, and lubrication that can partially help.
Personally, being a bit on the thick side, I've noticed that I experience a bit more tightness with a partner if I haven't had sex with her in a week or more, so there's some short-term loosening effect, but it's minor.
What do the women have to say about this?
FF
Seriously though, look into it.
