I'm not gonna lie; I saw 2 women sunbathing topless on BARMOUTH beach in West Wales the other week. So it's not just 'abroad.'
There was a 4 foot high wall alongside the beach, and they were about 3 feet away from it, with their stuff against it.
DH isn't usually arsed to walk much, but he walked along the wall 7 times, 'just for the exercise...' mucky little bastard 
Me personally, I did look (for a few seconds,) a couple of times, as I was curious, but even though I wouldn't go topless on the beach, (as I would be too shy to do so,) I have great admiration for women who are confident enough to do so.
Anyhoo, back to the point of the thread. The OP is not being unreasonable to feel a bit uncomfortable with someone showing boobies, coz she can't help how she feels. But she is being unreasonable if she thinks the woman is showy, and an exhibitionist.
Shagging with the windows open, when children are in next door's garden just to let people know you have a sex life! is exhibitionism; going topless on a beach when it's hot and sunny (particularly abroad,) is not.
Teaching your child(ren) that showing breasts is dirty or wrong, is not a good thing to do. And to say 'oh would you go topless when you're shopping in the supermarket?' is just plain bloody daft.