If you can see a woman's nipples through her shirt (that is they are pointed and hard) the nipple is not erect it is constricted, this happens when the body is cold (this is why wet tee-shirt contests work) the nipple of an aroused person is engorged with blood and quite soft to the touch. The nipple expands and the entire breast becomes larger.
Noding this because I had to explain the difference to a guy (28 years old and fairly world wise) a few days ago.
The fake erect nipples are especially disturbing when you consider this.
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