I think people are making a mistake by connecting the objectification of women with the women's personal choices (it sounds like people are confusing the concept of 'objectification' with the concept of 'oppression'). How is it relevant to the argument against the possibility of objectification in society, by arguing and appealing to the question of whether or not the women had a choice? I mean, a person could purposefully light a forest fire, or accidentally drop a match and cause one, but the result would still be the same, a forest fire. Objectification can still happen, despite whether or not it was the person's choice.
Do I believe Playboy contributes to objectifying women? Yes, it is not the sole contributor, but it plays its part, it sells women as images (things) to be viewed and consumed. Regardless of choice of participation, all of who they are as an individual, their thoughts, feelings, beliefs, etc. are not experienced through photographs of them doing poses that are only suggestive of one thing alone: sexuality. The photograph's primary purpose is to be a product for the magazine, one to sell women as objects of sexual desire to be consumed, an image, a thing that is not a woman, a representation of individual human beings. This can influence the overall concept of what a women is or should be, just as any other experience can influence an individual. And if the influence becomes prevalent and spread enough, then yes, it is quite possible for it to affect overall society.