You touch on another reason I'm not always comfortable saying "oppression by." For example, girls who are "circumsicized" are often pressured into doing so by the women around them. In this case, it is not even necessary for a man to be present for sexism to occur. Yet all of the women involved are oppressed, even if they are also employed as tools of that oppression. The ones who still benefit are men.

Another example might be minority activists cajoling one another to be more "mainstream" -- which is part of the message those people became activists to protest against in the first place.