The whole 'breasts aren't just for feeding babies, they're sexual' thing is interesting, because apologists for Page 3 often try to claim that it isn't soft porn, it's just good clean fun, appreciating the natural beauty of the female body 
So breastfeeding in public isn't fine because breasts are potentially sexy, but women's naked breasts in a national newspaper is fine because it's not sexy, it's a good old British institution, like deckchairs or fish and chips.
The reality, as people have pointed out, is that hands, mouths, penises, breasts etc are multi-functional and can be used for different things in different situations. Certainly when I'm breastfeeding, my boobs cease to feel remotely sexual and DH isn't allowed anywhere near them