I read your entire post, but I don't believe you have any legs to stand on.
Warhammer 40,000 is a game based on war, and is marketed to males. This is no different than a product marketed for women. Why is it always women putting on lipstick in the ads?
Reality is, humanity is divided into two sexes, and as such various aspects of society will eb geared to one sex over the other. Trying to androgenize everything is not the answer.
The answer is to not allow a 7 year old girl partake in a hobby that is marketed for 12+ year old people. I find it odd that you take the opinion of a child just barely realizing Santas Clause and the Easter Bunny don't exist.
The answer is to just cope with the reality that purses are for women, giant Ford Pick Up trucks are for men (the female models have vanity mirrors in them, just fyi), dresses are for women, and 40k is for men.
Now, if you were to pick an aspect of society that does not require a sex, say, space exploration or politics, and fault it for being sexist, then I would agree. But Warhammer 40k is a game designed for males.
Now, where is your letter to Mattel complaining that Barbie doesn't have enough males playing with their dolls?
- edit -
Space Marines - Male
Chaos Space Marines - Male and Female (yeah, more females here than Marines, go figure)
Deamons - Male and Female
IG - Male (the one or two female models don't count, lol)
Sisters of Battle - Female
Eldar - Male and Female
Dark Eldar - Male and Female
Tau - Male and Female
Tyranids - No sex (aliens)
Orks - No Sex (fungus)
In 40k, females are represented quite strongly. It is the perspective that GW's flagship army, Marines, is all male that makes the game seem 100% male, when in truth it isn't.