The idea of conformity is key to the theme, but Marge Piercy is not saying that women must first lose their identity in order to find one. It would seem, rather, that Ms. Piercy implies that a girl should focus on her attributes rather than finding fault with herself. Never should a girl conform to the ideas of beauty to the extent that she injures or kills herself as the "girlchild" of the poem does.
I agree that, though society places unrealistic expectations on both men and women, this poem really only deals with women--or in this case girls. The idea posted above is a good one. A little bit different approach is to discuss how early the expectations start, how easy it is to create trouble in this area, and how quickly one can fall prey to unrealistic standards of acceptability. The girl in the poem is very young when she hears a few careless words about her nose and weight. After that, she sees all things through the lens of a big nose and fat thighs. A possible thesis, then:
In a society which places high expectations on appearance, women are quick to accept the careless comments of others as truth and to take drastic actions in order to conform to its [society's] unrealistic image of beauty.
I don't think that you should do the his/her thing here. This poem is clearly a statement about what society does to women in particular, not to men and women both. Other than that, I think you're on target. I would say emphasize the gender specific nature of the poem and the damage that is done to women by our society. I'd say something like
"Our society objectifies women and values them only for their looks. Women are driven by this pressure to do destructive things in an effort to live up to society's expectations."