Why does cheating anger the partner?

i seen many occasions where it happen... from tv cartoons to real life situations... a guy is there having sex with his girlfriend then she screams out her exes name... and the boyfriend gets really mad at her and breaks her neck,

but i I don’t not understand this, why does the current partner of the person get so mad for being cheated on? I don’t get it,
if I found out a girl I love was secretly fucking someone else behind my back I wouldn’t be mad at all and wouldn’t wanna beat her up or anything, if Be very sad about it and feel like I’m not good enough for her, I might even cry
2 mo
Anyone else like me who would jus get sad instead of mad? Or is it jus me?
Why does cheating anger the partner?
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