No the permission for polygny is bound to certain conditions which no normal human being is able to fulfill to 100%. That's why Allah added the words like "if you fear"(see in [4:3][1]). 

Secondly it is human nature that a wife wouldn't like to share her husband and Allah knows his creation best. 

Finally scholars consider the condition of being the lonely wife added in a 'aqd nikah as valid. 

So there's no reason considering opposing this per se as kufr. Unless a woman says Allah is unjust by ordering or allowing this. 


  [1]: https://legacy.quran.com/4/3