DadDancer
I would go for the reverse sexism argument and if the petition had been for an alternate male model one day and female another or a page 4 dedicated to topless blokes
Two wrongs don't make a right. This is about objectification of a person and it's effect on wider society. It's demeaning to see men as sex objects too. Although as emc points out it doesn't happen very often.
Can I remind you:
Tits are not news
Taking tits out of the newspaper is not removing freedom of speech. Other countries have free press without sprinkling tits through their other 'features'. Germany got rid of their page 3 equivalent this year, perhaps because they realise it's old fashioned, unnecessary and creepy.
Would you be ok with your daughter modelling for page 3? Would it make you proud? If so, I think that's sad. I also wonder what happens to these girls when they grow up and perhaps are no longer able to model. Not everyone is Melinda Messanger or Katie Price. I can't imagine being taken seriously in my current career if someone found out that I'd been a page 3 model in the past. And I'm sure with the internet the way it is, it isn't easy to hide your past.
On another note, DH went to school with a page 3 model (The Sun waited for her to turn 16 and she did it just after her 16th birthday) and the teachers had taken the page out and put it up in their staff room with the following title... "This is what our students achieve" I don't think that they were proud. I think that this was said with a raised eyebrow. Although I expect a number of the make teachers 'appreciated' her talents. I obviously don't blame the Sun for the actions of teachers in the mid-90s, but these actions are not against the law, and perhaps they thought it was ok because this sort of material is freely available to all regardless of age. Also, a couple of teachers at that school were prosecuted for having inappropriate relationships with under age girls. This makes my skin crawl. That they may have been 'appreciating' a young former student who set a president for other young girls still at the school to be 'appreciated'. shudder I'm not putting any of this together in a "pedophile teachers slap picture of 16 year old to wank over in the staff room" sort of way. But because it's available, without restriction, it's ok to make a joke of. But then they're just expressing themselves and we live in a free society.
Would you be happy for your daughter's picture to be put on walls across the UK for people to 'appreciate'? I think my Dad, DH and other male relatives would be mortified!!!
Anyway, I digress. I don't get how taking page 3 out of a newspaper is infringing on freedom of speech?
And if I choose to go out topless, or naked, I could be arrested for indecency. How is it that it's ok to put these sorts of pictures in the Newspaper if I can't walk down the road with my breasts out? 
Can I ask you how you feel about public breast feeding. Merely because I'm interested :)