Should the word mistress/mistresses are/were associated to a lawful married second, third or fourth wife..Then,. Yes Islam allows having this kind of mistress/s.

However, the word mistress most commonly practice in media is for a ‘keep’ without any formal/legal relationship and should this be the mistress you are referring to than I state that Islam DOES NOT ALLOW this kind of relationship.

The idea of a declared marriage with witnesses in Islam as well, is the very reason to avoid any illegal secret relationship which in result can bring doubts and etc. that is totally a different discussion so I pause on this.

Having briefly stated the actual understanding from Islamic perspective, allow me to say if one of the Muslim or for the matter all the Muslims decide to break the beautiful defined discipline of marriage and instead adopt the idea of having mistress/keep/s….this shall be a wrong practice of those Muslim/s which ISLAM did not permit. Will they be classified Muslim and when they will be classified out of Islam and or what kind of penalty/ punishment is ordained to these practices is also a different subject and hence I pause again.

Hope this helps to understand the legitimate Islamic code on the relationship with women.

I in my best capacity avoid bashing any community, culture or an individual and will highly appreciate for all us to practice the same, as this helps to gain knowledge and share ideas peacefully..
Peace

Allah knows best and May He forgive us all for our intentional or unintentional mistakes, publically or privately…..