I think women experience saggy pelvic floors because we have under-developed ab muscles.
In my experience, including visits to a physical therapist, and 4 years of reading and study on this issue, the lack of core strength is what causes many women to have pelvic prolapse...it seems odd, but the only thing keeping women upright when their abs are weak, back muscles are weak, glutes are weak--is the pelvic floor.
So the good news is, the pelvic floor is so awesome, it can hold you up--but it cannot do it very well, and sure, by the end of the day, and any straining which causes intra-abdominal pressure to increase (i.e., the BM) will actively push the pelvic floor muscles down and out, and make the problem worse.
Carefully working on core strength, perhaps with the guidance of a physical therapist (because if your pelvic floor is currently working to keep you upright, there's a whole lot of ab-requiring moves that your body has found new ways to execute without using your abs) will allow the pelvic floor to do its job, and ONLY it's job.
It is possible for extreme birthing scenarios to cause trauma (i.e., forceps, 5 or more hours of pushing) and cause a cystocele, but I think for most women, core strength is key.
I recently read this book,http://www.amazon.com/Baby-Bikini-Mi...3152985&sr=8-1
which might win the award for Worst Book Title Ever, but the author of this book correctly instructs pregnant and post-partum women on how to properly tone the abdominals--simply DOING CRUNCHES is not the answer, as there can be a diastis of the recti muscles. While the author doesn't talk about the pelvic floor w/ his book, it's one of the most useful books I've read on ab strength.