Why on earth do people compare this to a man going topless? It's not the same thing, and it's so lame to pretend that it is. Women have breasts. They are seen as sexual. Men do not. It's like saying, "well, my 5 year old hasn't got a top, on so why do I have to?". It's not a comparison that makes any sense.
Personally, I find my own breasts highly sexual. I am aroused if topless. I would not feel comfortable at all getting my breasts out in front of strange men and kids. Just no.
Would I judge you, if you did? Yes, in all honesty I would. I would think that you were attention seeking and that it was highly inappropriate. If you don't want tan lines, you can move straps across. Of course, you'll get a load of women saying, "your boobs, your choice", but if their DH was on the sunbed next to your sunbed, I doubt they'd be so enthusiastic. Especially if you have great boobs, and theirs pale in comparison.
But this is MN, and often people don't say what they really feel.