Perhaps I should have explained the bio-mechanics of this a bit more clearly as people seem to be confusing what's happening here.
Without getting all biologist on you, all muscles act in one direction, the cells contract when they receive a stimulus and then relax when the stimulus is removed. In most skeletal movement there are pairs of muscle that work together to actuate movement, in say your hand there are separate groups of muscles for opening and closing your fingers. This allows very precise movement so you can type on your laptop and pick your nose.
Sphincter muscles do not have an opening pair, they only work to constrict the orifice and when they relax they open up but only due to whatever pressure they are containing.
This is the mechanism at work in the pelvic floor, the muscles can be trained to become stronger and close much more strongly but you cannot train them to open wide (like say you open your your mouth) as there is no muscles to do that.
So for instance during childbirth the cervix (not part of the PFM) relaxes so much that the pressure of the babies head is able to open it from a tiny point to 10cm or more. It is the uterine muscles that are contracting to push the baby down and out and as it passes through the vagina all the lower muscles simply relax to allow passage. The last barrier is the pelvic floor which is not used to being forced open to this degree even fully relaxed. As all mothers will tell you trying to relax during childbirth is hard which is why you hear all the cliched "relax, breath, relax, breath" then "PUSH" stuff. Pretty much everything down there muscle wise is concerned with keeping stuff in, when its time to let it out all you can do is relax those muscles as much as possible.
Now as
@SilverBlue questioned you can most certainly learn techniques to strengthen and maintain good control over the tightening motion but this will not allow control over how wide you can open.
The base closed state of the pelvic floor muscle groups (the levator ani) is when the muscles are contracted. Normally this is autonomic, meaning you don't have to think about closing your ass, it just does it itself. Where the muscles hit their "end stop" is important, as anyone who has suffered from cramps knows, uncontrolled tightening spasms in muscles are really painful. To avoid this the levator ani muscles learn their closed position and don't try to over-tighten.
And this is where my methods for resetting this end stop come in, by doing regular exercises that put the closed point in a much wider position the muscles "learns" this as their new default closed point and won't try to contract beyond it.
In fact as
@SilverBlue queried, yes you can indeed maintain strong PFM's and produce a gaping effect. You just need to make sure that when you do your Kegel exercises you keep something inserted to hold you open at the gape you require.
Now I've oversimplified a few things to make it easier to understand so if you want to dig deeper I suggest you google this, I'm also happy to answer any questions here or privately you want to be discrete.
