I have a good one for you! What does being pregnant and having a baby do to your body? Is it true that birth will enlarge your vagina, or make it "loose?" Does it get worse if you have more children? Is it noticeable to men? What about if you have a C-section? Are there other postpartum changes to a woman's body that affect how much she enjoys sex?
The harsh truth is that pregnancy and childbirth usually do cause physical changes (thanks for asking!), although these are by no means always dire or even particularly notable. The change you sound most concerned about is vaginal looseness and yes, it does happen. As I am constantly repeating, the vagina is not a fixed size like a train tunnel. It is a potential space, like a sock. Even so, it's supported by a whole complex of structures in the pelvis: not only muscles but also connective tissues of various types, all of which can get stretched out of shape, weakened, or even torn. Tone at the front of the vagina, where we feel most of the sexual sensation, can be lost due to perineal stretching, tearing, or the increasingly unfashionable but still sometimes necessary episiotomy. Nerve damage is fairly common too, and we need those nerves for more than just sensation; they also tell our muscles what to do. So while the sort of looseness that a million extremely crass jokes are built on may be rare, it's probably not as rare as the completely pristine and unchanged postpartum vajayjay. Change happens, and yes, pregnancy itself a.k.a. carrying a smallish medicine ball firmly lodged above your cervix for half a year is enough to do some of the changing.
There's an excellent if not particularly cheerful article called "Pregnancy, Childbirth, and Your Pelvic Floor: Understanding the Connections" at www.augs.org/custom/kb/answer.cfm?id=61. It's adapted from a book called Ever Since I Had My Baby (Random House, 2003), which sounds informative if a bit dispiriting. Do we really want to know that we might lose a fair amount of the sensation we enjoy during intercourse? Do we want to know how extremely common a little bit of urinary stress incontinence something we thought only happened to great-grandma really is? Actually, yes, we do. Much of the potential damage can be avoided or at least mitigated by good care and careful choices, so of course we want to know about these things ahead of time.
I looked up "changes after childbirth" or some such thing on About.com yesterday and found the usual sprightly lecture on doing your Kegel exercises. Under the "Did you find this article helpful?" heading was a large, crabby "No!," which cracked me up. I'm sorry the Kegels didn't work for Crabby Reader, but in truth they're about all we've got in our looseness-mitigation and restoration of continence arsenal. There are surgeries, but surgery is expensive and risky and requires the kind of recovery time that mothers rarely have available for lolling about on the chaise longue sipping sweet tea. In truth, a lengthy course of Kegels, energetically performed, can vastly improve muscle tone and help prevent its loss in the first place. Exercising your hoo-ha can feel undignified, but being afraid to sneeze (or laugh!) for fear of leaking is damned depressing. After all the Kegels there may still be a little extra space up there, but frankly, that can be put to good or at least entertaining use. It's the tonelessness toward the front that both partners can find dismaying and that inspires the jokes that end with (please forgive me, mothers everywhere): "Flashlight? Hell!