Even if only women should answer...

The female breast area is predominantly fatty tissue and is therefore not overly sensitive to pain. Of course it depends on the hand hitting, how padded, if at all, how far out to place the punch, how many times it has been hit, etc...
These are primarily felt to be blows against the 'honor' of the opponent, who can feel unsettled or even motivated to better defend her upper pubic area.
Single hits to the chest are no reason to give up a (boxing) fight. The BSA boxing and MMA fights offered by DWW around the turn of the millennium proved this.
(Exhibition) fights in which the chest area was described as a particularly desirable target area were carried out with gloves that were too lavishly padded to trigger painful feelings.
I am not aware of any real fight offers on the topic, topless or nude, leading up to (co-)submission of an opponent.
With regards, Frank

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PS: written with Google-translations-help GER > ENG