Why do wives and LTR girlfriends care more about validation from other guys than their guy?

When this happens, it's usually where the woman dresses plain and does minimal makeup around her guy, but when going around other guys, dresses sexy and does full makeup. What's the reasoning behind this? Is the SO just seen as low value to her?
Why do wives and LTR girlfriends care more about validation from other guys than their guy?
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