Why be in a relationship if he is going do that anyway?

He has a serious long-term relationship
Everyone knows about
They have a social media together

Yet he goes on a dating app
Where everyone can find him on it
And says he is there to date

I get it you get bored some day but you kinda know that before you start something with someone right?

I just don't get why the woman let him do that or even just stays with him.
Why wouldn't she already found it out and yet she stays with him?
Why be in a relationship if he is going do that anyway?
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