I was going to make this a more halachic piece but I'm on call in the local ER right now, it's an obscenely early hour in the morning and I don't have my seforim with me so my usual stylistic ranting will have to do.
Recently two Modern Orthodox schools, SAR Academy in Riverdale and Rabbi Haskel Lookstein's Ramaz school made waves within the Jewish world by announcing that girls who wished to wear tefillin during davening would be allowed to do so.
Naturally this has resulted in waves of outrage from the right side of Orthodoxy and waves of outrage at the outrage from the left side. On the right folks like Rav Steven Pruzansky and others like him can see nothing good in this. They note the lack of approval for this practice amongst the halachic sources and remain concerned that letting young women wear tefillin is a first step towards Reformativism and a meddling with the mesorah. For folks on the left any opposition to these girls is unacceptable and presents an exclusionary Judaism that they don't want to be part of.
Let's cut through a few things. Technically there's no issur to prevent these young ladies from putting on tefillin whenever they want. There is an issue over whether or not they can make a beracha since they are not obligated but these kinds of things have been argued about for years when women decided they wanted to start waving a lulav at Sukkos but nowhere does it say in Shulchan Aruch that women are outright forbidden to put on tefillin.
That doesn't mean the classical codes have nothing to say on the matter. While acknowledging the technical lack of an issur the Rem"a clearly states his discomfort with the idea. There are, in fact, no sources I could find while reviewing the subject over Shabbos that are comfortable with the idea or even say "Well, if she really, really wants to..." In other words, while the lack of prohibition exists the concept of women putting on tefillin is not recommended by the authorities.
Now for many on the left this is no barrier. They will point out that Michal, the daughter Shaul wore tefillin. They rarely point out the next part of the Gemara where Chazal said that the contemporary authorities protested against her. They'll point out that Rashi's daughters wore tefillin. Well, once your father knows as much as Rashi...
In fact there is no answer to give a woman determined to wear tefillin. She's going to do it and if a Rav pushes back she might use the rebuf to jump ship to the local Conservative synagogue. Certainly the Rav will be roudnly criticized by the relevant folks for daring to deny the young lady her "right" to get closer to God through the tefillin.
Leaving this aside one must then ask: why does a young lady want to wear tefillin in the first place?
Let me point out that there is one reason why men wear tefillin: because God said to. When I get up in the morning, before I daven Shacharis I put on my tefillin because I have to. If I decide one morning that the tefillin aren't doing anything for me it doesn't matter. I still have to put them on.
What's more, despite popular appearances tefillin aren't tied to the daily prayers. If one wakes up on a desert island one morning bereft of one's possessions one still must daven, even without any tefillin in sight. And in the reverse, if I decide I don't want to pray one morning I still have the obligation to put my tefillin on. Yes, we make a connection between the two because nowadays that's the only time we wear them but really one does not depend on the other.
Which brings us back to these young ladies. I have no doubt that they pray every morning with great sincerity and conviction. As far as their actual obligations go al pi halacha they fulfill them. So why the need to put on tefillin? They are not chayav in the mitzvah. They don't get the same reward as if they fulfilled something they are obligated to do. They annoy many people by doing it. So why the urge?
I will suggest that there is a simple reason: because the boys do it. Remember what I've been saying about the role of ritual in Judaism nowadays: it's everything. The more rituals you do, the more Judaism you're doing. When a boy goes to pray he puts on his tefillin. When a girl goes to pray she just gets to open a siddur. In a ritual-centred model of Judaism why wouldn't the young lady get the impression that the boy is doing more than her? If she sincerely desires to have the best prayer experience possible why wouldn't she see adding tefillin to her prayers to bring them up to the same level as the boy's?
The actual halachic position is simple: a girl without tefillin and a boy wearing them are both in the optimal position for davening.
The perceived ritual position is also simple: the more you do, the frummer you are. Until that perception is changed one cannot argue against the "Morethodoxy" folks and their desire to change Judaism to suit their underlying secular liberal values.
Once again ritual has taken over reason and become the defining feature of our practice of Judaism. That doesn't mean it's correct.