I tend to agree with Jugner's first post in that men are partially to blame, but so are women. It's a balance between the horniness of men and the vanity of women. Men will naturally tend to see women in a somewhat sexual context due to hormones and general masculinity. Women, in turn, succumb to and perpetuate this image because of their social nature and because so many of them like to be put on a pedestal. Understanding the male side of this equation is simpler, because as men, we're pretty blatant in our libido. The female side is a bit more subtle, so that's why I'm going to elaborate more on it.
In my experience, women seem to have a love-hate relationship with attention. They want to be noticed and emotionally nurtured, yet, they often go through moods of wanting to be alone. While the same could be said of men, the rationale behind women's behavior in this avenue is largely hormonal and societal. Whether or not they admit it, most women seem to enjoy being "sexually admired" -- as long as it's from the right guy. If a woman (who isn't married or in a serious relationship) is getting "checked out" by a guy she finds attractive as well, she's usually going to like it. By the same logic, she's going to probably dislike this same attention from a guy she isn't attracted to. Granted, in the case of bisexuals and homosexuals, you can add or replace other women to the equation.
I know this sounds like common sense so far, but here's where it gets tricky. Some women are truly exhibitionist; I've known plenty of them myself, and there are probably at least a few who even frequent this site. The reasons for this desire are varied, but overall, exhibitionism can be most concisely explained as the need to receive attention from all members of the opposite sex (and in the appropriate context or orientation, members of the same sex). Some women literally enjoy the idea of having a bunch of horny men (who often pay well) oggle over their body. Granted, I'm not saying that the majority of strippers or models feel this way, but there are enough of them out there to bring up this idea. With modelling in particular, it's a matter of vanity. I think it's pretty obvious that someone like Anna Kournikova was probably brought up in an environment where she quickly concluded that she was extraordinarily attractive. While some women don't like the kind of attention this entails, plenty of others do. I've known several very attractive women that have aspired to be models because they are well aware that they were blessed with the kind of physique that such a job requires. I honestly don't think most of these women who desire the kind of sexual attention that modelling and stripping involve really think about the societal repercussions of their professions. When a model goes to a photoshoot, the thought of a teenage girl starving herself to look like her in these pictures subsequently is probably the last thing on the model's mind.
Much of the societal pressures to look a certain way among women comes from women themselves. People always say that men are very visual, but women are too; they just focus on different things. Men often look at a woman's "assets," while women usually look at a guy's clothing. Yet, this female focus on clothing extends to other women (even in the case of straight women). Women are usually the ones that drive the fashion industry in a particular direction. Most fashion designers are women; granted, there are plenty of significant male designers as well, but if you'll notice, a lot of them are gay (which almost gives them a female perspective).
To explain these pressures in more detail, I offer the following blunt observations.... Most guys I know make a significant distinction about what they find attractive and what they don't. There are obviously varying degrees of attraction in our personal perspectives, but for the most part, we either find a girl attractive or not attractive. Sometimes, "not attractive" crosses into the category of "ugly," but in general, most girls that fall into "not attractive" are just plain or maybe a bit below average. Quite frankly, most American males don't go for fat women, for example. The problem herein lies the discrepancy between how men define fat and how women do. You'd think it would be a simple state of comparison involving body types, but once the female perspective comes into play, it gets a lot more complicated.
Many women I've known seem to define fat as anything that's wider than the girls you see in Maxim or in catalogues like Victoria's Secret. Granted, I realize that plenty of women are more sensible or realistic than that, but you have to take this perspective into account to understand much of the teenage female point of view. High school is obviously a very socially oriented environment. From what I remember of those days, kids seemed to judge each other by petty things like clothing, what car you drive, and general appearance. From what I understand, teenage girls tend to be even more judgmental toward each other in appearance and clothing than guys are. When you combine this tendency with the images presented by professional models, you end up with a lot of girls that develop eating disorders in order to "fit the mold." Ironically, many of these girls are already pretty close to the "thin ideal," but their insecurity leads them to think that they are fat.
Meanwhile, most of the teenage guys already know what they like when it comes to appearance, and many of them don't make much of a distinction between anorexic Ann and that model they oggle over in the latest Maxim. Quite frankly, what it often comes down to is a higher standard that is set for the girl by her female friends than the one that the guy has. Most guys aren't gonna care or even notice a 10 pound difference in their girlfriend, but the girl's friends will. What I'm trying to say is that unhealthy lifestyle habits among women often originate from images created and supported by women themselves. Once these habits and perceptions develop in high school, they often linger through the adulthood of many women.
In conclusion, it really is a matter of mostly women judging each other too harshly that leads to much of the "objectivizing" of women. Men obviously play a part in this, but we're generally less picky and scrutinizing about it.
As a final note, whether or not such objectivizing is a sin is dependent on your definition of sin. In my mind, a consensual act is not a sin. I've noticed that a few seem to have brought religion into this discussion, and for those of you that have, I have one question: why would God, of all things, care if you masturbate to porn? Why is pornography sinful? Lust may be considered one of the 7 deadly sins, but I believe the original context of its mentioning in scripture referred to things like rape. You can argue that premarital sex is sinful, but I'm pretty sure God (if he/she exists) doesn't partake in judging people by thought crimes.