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 "as much as you can". 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 this.

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). 

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.