Feminists struggle for equality, an end to gender based discrimination, stopping male on female violence and rape, that sort of thing.
In our society, which is not equal, and where women are subjected to sexual harrassment by random men from puberty through to menopause, with little recourse, page 3 is not fine. It perpetuates the idea that the point of women is for men to look at, that a woman's worth is nothing more than how aroused she can make a man feel, it makes many women feel uncomfortable when they are simply going about their daily business.
I wonder, the people defending it, would you honestly be really sad to see it go? Would it spoil your day not to be able to have a look at some breasts in your morning paper? Why does it mean so much to you?