This isn't true. One cup of cooked quinoa has about 1/10 the calcium of just 1 cup of cows milk.
From the USDA nutrition database:
Not to be entirely negative, there are tasty vegan calcium sources that do match and exceed the amount of calcium found in cows milk:
“May all sentient beings be free of pain and suffering. May all sentient beings experience eternal joy and happiness. gate gate pāragate pārasaṃgate bodhi svāhā.”
At least it doesn't get repeatedly impregnated and have it's babies stolen and murdered.
And... there are much better sources for calcium in a days diet than babies milk.
Don't let the perfect be the enemy of the good
Vegetarians are one of the groups I've heard the most untrue, anti-science urban legends from. Theirs are not just about food either--pick any "natural" topic such as alternative medicine and it's just nuts. For some reason this forum seems to have more sensible critical thinkers than most.
Feminists? Never heard an urban legend from feminists myself.
If people hear a figure or concept that seems to support their discussion particularly well it can start pinging around in ideological communities and become a kind of meme before anyone puts the brakes on it. I don't see vegetarians as being quite as bad as you do, but the health food types can end up in odd places. That and a lot of veg*ns overstate the health benefits of the diet, which isn't to say it's all wrong, but the truth is more complicated.
I happen to belong to League of Women Voters which is not anti-men, has male members and don't say crazy stats...in my experience anyway. So thankfully all feminists are not dishonest.
Dishonesty and willingness to spout unresearched statements as "facts" discredits both the individual and the cause s/he promotes.