Although it is de rigueur these days in Western literature to assume that gay men are born gay (and I'm not disputing this), yaoi still, to an extent, assumes an environmental cause.What Did You Eat Yesterday? (which isn't really yaoi) takes the former position. Shiro and Kenji have a poignant conversation where Shiro admits to being tempted to bely his gay nature and enter into marriage with a woman: in his heart of hearts, Shiro wants to be the classic salaryman.
However, he admits, he would no doubt have strayed, ultimately causing great pain to the people about him.
A great deal of yaoi, on the other hand, rather than seeing orientation as an inherent characteristic, falls back on the influence of environmental factors--indifferent parents, poor role models, cruel prior girlfriends.
However, the treatment is not quite the same as the Western treatment of environmental causes. That is, the point is not to undo those environmental factors or even to blame them. The manga/yaoi approach is more similar to Deborah Tannen's That's Not What I Meant--although she argues that the differences between men and women in terms of communication come down to social training, her focus is, "How do we deal with this? How do we communicate within the culture that we have?"
Likewise, yaoi may posit environmental factors--but, well, who cares? The focus is on survival and adaptation. Characters are more likely to ponder their lives like Shiro, no matter their personal beliefs. Revolt is a very different thing from the (seemingly constant) Western attitude that society NEEDS TO CHANGE RIGHT NOW BECAUSE I NEED IT TO!
The yaoi perspective is refreshing, being grounded in day-to-day life--and what human beings/human nature can actually handle.
