In some conversations I've had with people about the whole "Where does the slacks = immodesty notion come from, anyway?" topic, I've realized that there's a common thread running through a lot of the "But the proper holy Catholic way is..." types of thinking going on out there.
Bear with me; I'm thinking out loud, so to speak, so this isn't going to be all that orderly.
What are the various points of contention among Catholic families as to how to live, and how to raise our children? Some of them are big deals, such as homeschool/Catholic school/public school debates, or some TV/no TV except EWTN/ no TV in the house debates. But on these big debates, oddly enough, people seem to be willing to accept that different families may arrive at different conclusions much of the time, and that a family who tries homeschooling and finds it doesn't work for them isn't necessarily abandoning their children to the prevailing culture, or that a family who occasionally tunes in to "Little House" reruns isn't handing their children over to Satan, etc. Moreover, there's more patience, seemingly, for where individuals and families are on the journey--that perhaps they are being called slowly to consider homeschooling but are still in the exploratory phase, and that we should respect that, be helpful, but not pushy or overbearing as we share our homeschool experiences.
So the "big deal" matters don't seem to get as much open criticism, perhaps because it's easier to know that making these decisions isn't always easy and doesn't always work for each family's situation. Why all the contention on the smaller matters then? Why so much insistance that dressing a certain way, praying a certain way, following the liturgical seasons a certain way (with rules about when to put up a tree and why we shouldn't mention Santa and all sorts of other issues), going to a certain parish or a certain Mass, covering one's head in church if one is female, and so on?
Some of it, as many people have said, may be because so many of us were taught by the loosey-goosey, anything goes religious educators in Catholic schools or parishes, who told us--wrongly--that all sorts of things the Church actually teaches were really no big deal. I can remember hearing that if we hadn't killed somebody, we hadn't committed a serious or mortal sin, for instance--and this was during my adolescence, when serious sins become quite possible (and not just the ones violating a certain commandment; we never learned that missing Mass on Sunday without a good reason was a sin!). So, as several wise women I know like to say, we don't quite trust our own impulses, and when somebody shows us a path to holiness that looks doable and not too complicated, our desire to embrace these things comes from that place of early mistrust, to a certain extent.
Some of it, too, may come from a tendency to lump everything that is seemingly "good" into one big category, without differentiating between greater and lesser goods. So we see things like daily Mass attendance or a daily rosary/other prayers as being on the same level as wearing a veil or dressing in skirts only, without realizing that all of these things are on different levels: Mass attendance is the greatest good in this list, and wearing skirts probably last, but we start to see them not only as equally important, but as practically required for a serious Christian.
And we may also forget what the high school religion textbook my family used when I was in high school was careful to point out: even among good things, there are specific aspects relating to one's own vocation which must be taken very seriously indeed. The text used the example of the wife and mother who attended several daily Masses and spent hours in prayer in front of the Blessed Sacrament, but who was thereby neglecting some of her duties to her home and her family. Her notion that the good example she was setting for her children outweighed her neglect of some of their material needs was truly mistaken, and she was not pleasing our Lord by her actions, because the vocation which she was supposed to be living was not that of a contemplative nun, and meeting her family's needs was the highest and best form of prayer she could offer to God.
So some families at some times may not be able to attend daily Mass, or to pray the rosary, without neglecting their little ones or placing a hardship of some other sort on the family. And some mothers might not be able to wear skirts exclusively while still doing all their chores (bearing in mind that the decision to wear skirts as a voluntary penance is fine, but that skirts themselves don't produce holiness); and a mother whose attempts at wearing a head covering at Mass produced nothing but anger toward the toddler who kept snatching the hat or veil off of mommy's head might want to rethink the whole thing.
Because the externals of our lives don't produce holiness. Attending daily Mass is definitely productive of grace--but not if we spend the whole time distracted or frustrated or otherwise barred from active participation. Praying the rosary is a beautiful devotion and a good prayer habit--but not if we have to skimp on dinner preparations and buy convenience foods every day in order to have the time to say it. Covering one's head at Mass can be a lovely act of voluntary penance--or it can be alternately a temptation to pride and a distraction. Wearing skirts as a similar act of voluntary penance can be productive of grace, too--but not if we're already inclined to wear them and go around looking down at those who don't, or worse, judging them guilty of sin for not doing what we're doing.
The Pharisees in the Bible were famous for focusing so much on the externals that our Lord called them "whited sepulchers." On the outside, they radiated 'holiness,' but their hearts were devoid of love, mercy, humility, kindness, and joy. Sometimes, traditional Catholics are wary of being compared to the Pharisees, because the liberal Catholics liked to use this comparison as a way of insinuating that people who actually cared about what the Church really teaches, etc., are Pharisees. But there's a big difference between wanting to grow closer to God by obedience to His Church, and by thinking we can grow closer to God solely by what we do on the outside.
How do we know the difference? It's simple, really: we are being Pharisees when we make the externals more important than our interior journey toward holiness and when we begin to judge others and look down on them for not adopting our voluntary habits and practices. Being concerned when a Catholic friend skips Sunday Mass regularly in order to go to brunch with friends or when she goes shopping in a mini-skirt and halter top is not being a Pharisee; being concerned when a friend says that she can't get to daily Mass more than a few times a month or because she wears slacks while crawling around the floor after an active set of toddler twins most likely is an indication that we're veering into Pharisee territory.
Scripture tells us that we must rend our hearts, not our garments. So long as we let our interior castle fall into disrepair it doesn't matter if we wear the skirtiest of skirts or the most floor-length of somber black veils; our Lord sees the inside, and knows us for who we are, and calls us, continually, with His great love, to turn to Him in humility and penitence.