Breasts are sexually stimulating to men for two primeval, subconscious reasons. A woman with large breasts, in our primitive brain, means that she will be able to amply feed offspring. It is about procreation. The same goes for the shape of a woman's hips. Wide hips mean easy childbirth. It is about procreation.
Also, there is a Pavlovian effect. Men are conditioned such that when they see naked breasts, sex is about to happen. In my experience, every time I have found myself involved with naked breasts, something good was only minutes away. Now, my brain is wired that way.
How do natives run around topless all the time and nobody cares? Because they do not have that experience. Breasts do not equate to sex to them like it does for most Western men.
The most breasts I have ever seen was in Palma de Mallorca Spain. The beaches there are clothing optional. I was sitting at a cabana with a patio and had to chuckle a little. Right outside the patio were the showers. LOL So, all the women and girls would come and shower in front of the patio, which essentially was kind of sexy show while you are drinking a beer or whatever. And the women did come. They were anywhere from 10 years old to 75. Pregnant women, teenagers, mothers, sisters, all kinds of women came there.
I hate to admit this in front of other men, but really, after an hour or so it got boring. Once my primal brain realized that this particular exposure to breasts was not related to imminent sex, I lost interest. It was interesting for a while though. It is something I have never forgotten.