Well,
the difference between black and gay people is that black and gay people is that these are not behaviors (yes - gay people have gay sex, but what makes them gay is their attraction to the same sex, which is not dependent on actually having sex with the same sex).
But my critique with this article is that I would guess much of the correlation between fat friends and yourself being fat is that for the most part, fat people and skinny people do not regularly hang out. I will grant you that among men, there is more a prevelance for skinny guys to hang out with fat guys, but often, you will see a group of skinny guys with the lone fat guy.
As for women, forget it. Women see themselves in part through their friends. This is especially true in younger women. For an example, look no further than the Sorority vs the Fraternity system. Most sororities are either thin or fat. But fraternities will always have their "moose" members who because they played football, or can drink a lot, are allowed into the circle of thin people.
Now, I grant you that even amongst many thin groups, there has been a decline in standards of what is appropriate weight. So these groups often gain weight together, as they often eat together, and as one couple gains weight, other couples feel more comfortable.
But I bet most of the correlation between fat and thin friends is due to the above segregation factor, not that one fat person enters into the think circle, and poof, we all blow up.