To take your first point, the main point of difference, fairly obviously I would have thought, is that women have external genitalia on their chest, men do not. Whether that makes any objective sense is a slightly separate debate but suffice to say if I decided to walk down the street with my nips out it wouldn't be seen as acceptable, I'm not sure it's even totally legal? Even if you say you would have no issue with women walking about topless and not bat an eyelid you only have to listen to people who tell you how disgusting it is seeing women BFing in public to accept that there IS a difference in perception!
That aside, the other difference is that society contains a certain amount of structural sexism, which is dictated by and supports male privilege. Men have not had to fight to have their own bank accounts when married, to work outside the home, to be able to vote, to be taken seriously as just 'people'. Women meanwhile, are still represented daily in a national newspaper as the sum of their bodies, as a bit of harmless fun for male titillation, whilst a sub ed puts words in their mouths that they didn't say to back up the day's editorial stance. (We don't want to know what you actually think love, just get your tits out and smile to cheer the men of the country up.) Whilst page 3 may not be totally responsible in itself for rape and DV and women being sexually and verbally harassed, it contributes to a culture that presents women as sexual objects who want nothing more than to be admired by men. When the model on page 3 stands there smiling asking for men's approval of her tits (are they nice enough for you sir?) it's hardly surprising that the man shouting 'nice tits' at the woman over the road thinks he's handing out a nice compliment. So what if I shouted 'nice pecs' at some bloke I see over the road? Would that be the same? Of course not!
As to the victim thing: I think you have misunderstood the problem that people like me have with this culture. I don't take issue because I am worried for the poor models. Admittedly I don't like the fact that they are having the piss taken out of them in News in Briefs, but I totally accept they have chosen to do that. However, what does get me is that by extension the Sun is having a massive piss-take about the whole of the female population. I haven't chosen to model for page 3 and I don't want to be reduced to a pair of tits for men's entertainment, with the 2 scenarios that if I get them out I'm thick and if I don't I'm fat and ugly and prudish.
Finally, your comparison between Miss World and Mr Universe is not like with like. Miss World is/was (does it still exist??!!) a 'beauty' competition; Mr Universe is a body building sport event. I'm not remotely into body building so won't pretend to know anything about it but when those men take their clothes off to reveal their bodies it's not like some male models being voted on who is best looking - they have engineered a very specific physique using weights in combination with diet to, as you say, achieve muscle mass. Given women practise body building too (is it actually called that, I have no idea!) I assume there must be a comparable competition. But that is not Miss World. There is no male equivalent to that, just as there isn't with page 3. Because we have male privilege and structural sexism.
And for the record, I don't think a woman taking her top off makes her intellectually retarded, but the Sun would have you believe that. Why else make a joke out of attributing some words about Cicero or the large hadron collider to her? I don't think it makes her 'morally retarded' particularly either, but I still regret her choice to do so for reasons I've already stated.