If women started going topless all the time after awhile it wouldn't even excite you.
The whole point of covering up is taht when you see what is typically covered it's exciting.
Breasts in and of themselves are nothign particularly stimulating to look at... it's that they are always covered that has turned them into something people get aroused by.
In africa, where women walk around with no shirts on all the time, you don't see the men walking around with boners 24/7... they are used to it and it's nothing special to them.