Although, over the course of my life, I've heard the familiar comment about how girls and women make their ladies' rooms trips together; until now I haven't really asked myself why.
The most immediate situation that comes to mind is one I experience just about every week, when my sister and I go out for lunch and shopping. Although there are times when one of us wants to go "wash our hands", and the other decides to head outside and get a little fresh air while waiting; most of the time both of us just decide on a convenient time to make the ladies' room trip, and then go there together.
With two girls or women, there's just a continuity in staying with the person with whom you have chosen to "hang out". There's this kind of thinking, "First, let's stop in the ladies' room and comb our hair and wash our hands. Then let's get our coffee. After that, we can go to the gift shop." What I realize tends to happen is that my sister and I have in common the first phase of our trip (and that includes the usual ladies' room trip to comb wind-blown hair and wash hands, as well as taking care of any other matters). The second phase of our trip out often includes separating and doing miscellaneous, individual, errands. We then semi-reunite by ending up in the same grocery store, going through the aisles independently, and then joining up after we're both finished shopping. In other words, there's kind of system to the whole "hanging-out" thing, with the pattern being "in common tasks", "not-in-common tasks", "sort-of-in-common" tasks, and then "in-common tasks" again (which leads to the shared ride to the non-driver's home).
Upon thinking about it, I realize that when two or three women get together they tend to stay together (probably because that's "the whole idea of it"), unless they have reason to split up.
I think many women friends share a different kind of "mental intimacy" than men friends do. Women have a closeness with other women. At its worst, it's the thing that is responsible for the "cackling hens" image. At its best, however, it is the thing that makes two little girls seem so enthusiastically engaged in relating to their friend(s); and it is the thing that even grown-up women discover they often still share with other women.
Women are often more sociable than men are. Girls and women will make a trip to the restroom as much a part of an outing as anything else. Most of us recall the inevitable drama in the girls' room during school dances. One fourteen-year-old girl was always crying, as her friends hovered (like hens) and commiserated over the fact that some boy had chosen to break up at the dance. In less dramatic times, the girls' room at a high school may have rows of self-obsessed and insecure girls, worrying about their hair and make-up in front of the multi-person mirrors. Even elementary school girls enjoy a break away from class and the chance to simply talk freely with their friends.
Grown women may not fret over their hair and make-up in the ladies' room, but they still share conversation and a common purpose - whether that's to wash their hands, comb their hair, or empty a bladder. Sometimes there may sharing of tissues (if the ladies' room has no toilet tissue). Sometimes there is sharing of hairspray. Ladies' rooms are often decorated nicely, so women often comment on that (or even just enjoy the prettiness of this special sanctuary). When a ladies' room is less than pleasant, they get to complain about that to one another.
In other words, while most normal women don't particularly love ladies' rooms, they don't hate them either. Perhaps we learn in elementary school that restrooms allow for having, or continuing, a conversation. (Why stop a conversation just because someone needs to empty a bladder?)
In a group situation, the same type of things apply; although there may be something else in a group. If there is a group of people at, say, a restaurant table, being the one person to leave to use the ladies' room can feel a little odd to women. Don't get me wrong. Women will go to the ladies' room alone if it seems to make sense. Also, it isn't that being the only one to leave the group is a big deal either. It's just that, I suppose, planning to make the trip to the restroom with at least one other woman makes a woman feel less as if she's leaving the group for some negative reason.
Whether we realize it or not, women have often learned that leaving a group alone may bring on the concerns of others for some reason. Someone may way wonder if she's upset. Someone else may wonder if she's sick or something else is wrong. It may even be on some subconcious level, but women may sense that if they leave as a loner there could be some misunderstanding; while if they leave as a "team" with another woman, it will be "established" that this is simply a trip to the restroom.
Some people suggest that women go to restrooms in pairs and groups because they're worried about freaks hiding in the stalls (and I suppose there could be some subconcious, or concious, aspect of this). I don't so much think, though, that women make the trip together primarily as a way to keep physically safe. I think it may more often be a psychological/sociological defense mechanism.
Women, too, may actually enjoy taking that little break away from a mixed group of people and being able to have that little moment of relating and "mental intimacy" with another woman or a couple of them. In a situation, such a trip offers a break from any tensions that may exist by being out with guys they don't know very well. When long-time couples are out together the ladies' room trip can give women that little break from that "same old guy". When the women don't know one another well even that quick little run to the restroom can create the feeling they know each other a little better. When they're old friends it gives them a chance to say whatever it is they couldn't say in the group.
For most of us, the bathrooms in our childhood homes were kept attractive and clean by our mothers (or other adults). Some ladies' room are, of course, nicer than others; but, in general, people responsible for public restrooms tend to try to make ladies' rooms kind of nice. Maybe girls and women just become accustomed to the idea that restrooms and bathrooms are often attractive rooms designed just for women; while boys and men may learn that public restrooms are more utilitarian and often unattractive.
Boys and men may have grown up in homes with attractive (or at least home-like) bathrooms, so perhaps the contrast between the home bathroom and the public restroom (with its rows of urinals) is enough to make boys and men see the restroom as something that simply does not encourage bringing socialization past their doors. Maybe too, the set-up in many men's rooms is such that it makes men feel just a little uncomfortable and only to glad to do what they need to do and leave.
Another guess about why women just naturally ask a companion to join her in the restroom is that women are of the same gender that their mothers were (needless to say). There's a natural "comfortable-ness" that women tend to share, and it probably has something to do with that. Fathers can be wonderful and loving parents, but when all is said and done, there is that thing that goes on between mothers and their babies and young children that does have a lasting impact on how that child will eventually relate to other people.
Some boys do grow up to be generally more comfortable with women than other men, but the rules say they can't invite a woman into the men's room with them. Besides, they probably wouldn't really want to. For boys and men who grow up feeling most comfortable around other men, it's as if they have branched off into their own gender at a certain age.
Things are just simpler for girls and women. They become very comfortable with, and close to, their own gender young; and they never have to branch off into the world of another gender. Instead, they "add on" relating to men on top of their lifelong feeling close to other girls and women.
Women just don't view the ladies' room as a place only for emptying the bladder. Ladies' rooms have pretty smelling soaps, clean paper towels, baby-changing tables, and sometimes lots of mirror and counter space. Women associate ladies' rooms with fixing their hair, touching up their make-up, pinning up a fallen hem, changing a baby, or taking care of any "monthly" matters. In other words, ladies' rooms are for all kinds of ladies' matters.
So, when one woman invites another to accompany her to the ladies' room, it's almost as if she is offering her companion a chance to step away and take care of any ladies' matters without having to interrupt socialization completely. It's almost a kind of "ladies' etiquette".
Most women will tell you that they visit ladies' room alone far more often than they do with companions; but when with one or more other women, it's just kind of a polite and friendly thing to visit the ladies' room together.