How to talk with her?

I have a girlfriend , we hangout a lot at any place...
Yesterday we went to buy clothes for her , she wanted to buy a tight shirt revealing cleavage... I did not like it because i do not like anyone to stare at her and do not feel comfortable with that... I told her that it is revealing parts of your breast but she responded that is my life...

It is so weird how she wants to wear these kind of clothes and doesn't want me to be angry

How can i tell her what i feel , i am not comfortable with that at all
How to talk with her?
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