A good rule of thumb would be whether women are displayed solely as sex objects under the control of men. Clearly, if a woman uses her beauty or sexiness to sell a products where she is a willing participant and is not presented or treated as powerless and obedient to men, she cannot be deemed degraded. If she is voluntarily participating in a portrayal of a women who is in control of her body and not presented as inferior to men, the question of degradation doesn't really exist.
Another factor would be the intended effect of the ad on the viewer. Does that ad promote disdain, disrespect or domination of a woman? If it does, then the ad demeans or degrades the woman, whether or not that was the intended effect. But what about a beer commercial during a football games that features big-breasted, curvaceous women promoting drinking as a cool and sexy in a flirtatious manner. Again, there is nothing degrading about women promoting beer to male sports fanatics unless the ads correlate beer and woman as a recipe for men having sex with women.
Perhaps the most iconic use of women as sex objects is the annual Sports Illustrated Swimsuit Edition. Obviously, men buy that issue to look at scantily clad women. While the pictures are extremely sexy and almost completely reveal a woman's body, the photos are taken in an artistic and respectful way. Sure, the idea is to capture some of the most beautiful women in the world in provocative swimsuits and poses, but there is no hint that the woman are being subjugated in any way to participate in the photo shoot, nor are they shown in photos being controlled or subservient to men.
In sum, the use of sex in ads to sell products does not degrade women so long as they are not presented as objectified beings under the control or domination of men.