>they can tell us themselves if they feel this way

Yes and they do. I am not sure if you realise this but this actually what the female author of the article says:

"I also have grave concerns regarding activities and events that are for “girls only”. These events instead enforce the stereotype, segregating girls and giving them (and everyone) a message that they are different. It is a band-aid attempt at a solution to a problem that starts at a much younger age, and seriously neglects those girls (and women) who don’t want to be segregated (many of whom never even come out to such events, and whose voices are sadly never heard). Children do not grow up into a segregated world, so why are we reinforcing that message? Why are we not just creating a variety of activities that may appeal to boys or girls or both? I can see taking issue with creating explicitly sexist activities, but there is nothing inherently sexist about having activities that may attract more of one gender than the other, especially when we make the available to everyone. "

The fact that women participate in these events is an obvious counter-example.

And yes, the author is essentially making your point. But she is also speaking from a position of privilege, as someone who was never intimidated by "boy stuff" and who admittedly got along better with boys rather than girls. I'm not sure her point is representative of the target audience of these kinds of gatherings, so it should be weighed with that in mind. Also its interesting to note that her comment was not in the first person, as in "this is how I feel about these events". It seems like she's doing what you are doing, projecting her ideas about how these girls will/should feel onto them.

Secondly, I would argue that your list, from my point of view, denoted 'racism, sexism, etc' is more rigorously defined as 'discriminating based on anything other than merit'.

Given that assumption, discrimination by race or sex for any reason would be wrong.

With those premises, I stand by my original conclusion that discrimination on the whole is wrong.

My comment nextdoor describes my problem with these sorts of degenerate defintions: http://news.ycombinator.com/item?id=5350842

To put it another way: if your definition of sexism means that women-centric events designed to foster interest in an otherwise underrepresented field is considered sexist, then its your definition that is faulty.

Here's a definition that doesn't strip the salient context from it (dictionary.com)

1.attitudes or behavior based on traditional stereotypes of sexual roles.

2.discrimination or devaluation based on a person's sex, as in restricted job opportunities; especially, such discrimination directed against women.

It's amazing how these discussions always devolve into cherry-picking definitions and parsing language like a machine to prove respective points. If you have to resort to that, then just bow out gracefully as you are missing the point of the discussion altogether.

To put it another way, the ends justifies the means. And I think you'll find plenty who would disagree with that statement.