'I'm not a feminist, but...' is usually followed by an extremely normal, acceptable opinion on womens rights. So why the prefix? The only other time you hear this construct is usually before someone tells you that they're not a racist, before launching into a tirade about the Islamic religion/amount of black people on Eastenders/last time they had a white taxi driver.
That's how bad some people believe it is to:-
a) be a feminist.
b) give an opinion on the treatment of women.
Yet these people, male & female, quite clearly have an interesting insight on such issues, which they themselves validate as being a feminist opinion by declaring themselves specifically NOT a feminist but the point of view they are about to express as being so with the prefix of 'but'...
Seems simple, no?