The article you posted has some pretty thoughtful responses to the question you pose.
I am a 30-something vegan man who competes in sports regularly. I find that the social expectation to eat "manly meals" is very real. If not for my strong sense of self, ethics, and my upbringing in alternative cultures, I might submit to the pressure when I am with my other athlete friends and competitors.
I am a vegan for a variety of reasons, but ethics is the main one. I have found that most vegetarians, by comparison, do it for health reasons. I have also found as a masters (adults) swim coach, most people swimming for health are women. There is social pressure for women to fit into certain social molds, not least of which centers on body image. That pressure, besides creating a template for unhealthy relationships with food and weight, can also provide pressure to care about health. Men, on the other hand, have pressure to be "manly", which is often equated with reckless behavior, or at least not caring about health in the same way as women. To top it off, "health", for men, often means looking like a football player, and being aggressive. Mistakenly, those two are equated as animal-protein diets. As a result very few men, even those looking for "health", go vegetarian.
In sum, I think that this 2:1 ratio is caused mostly by social factors.
In my experience, the reason cited most by women to go vegetarian is health or beauty related.