Where is the proof that a man can marry a second wife without the first knowing?

Question

I have seen some Islamic lectures that say that the man does not need to ask his first wife's permission, but also the first wife does not even need to know? Isn't this lying and deceit? How does such a thing become valid when it comes to a man?

I understand the proof that he does not need the permission of a wife is the fact that the Qu'ran makes it his right to marry more than one. But is it valid outside of the knowledge of the first wife?

Also does it count as unfair if the second wife knows of the first wife, and the first does not?

Answer 1

Indeed, it is deceptive to marry a second wife without the first wife knowing about the second (or without the second wife knowing about the first). However, scholars consider such a marriage would ordinarily be valid. Scholars acknowledge this, e.g.:

Lying and deception are, without doubt, two of the major sins and from among the enormities (kaba’ir). But again, they are independent sins, and do not invalidate a man’s second marriage.
_{Darul Iftaa}

Basically a marriage can meet the requirements of a valid marriage. JustAskIslam lists these requirements as:

The conditions of marriage are four:

1. The presence of the couple

2. Their approval

3. The presence of the woman’s guardian

4. Two male, Muslim witnesses

Generally, having more than one wife itself is discouraged (let alone doing so deceptively):

It is generally not advisable for men to marry more than once in our times, because of the harm and wrong that inevitably results, unless there is a genuine need.
_{Darul Iftaa}

And, even in Muslim-majority countries, there's also non-sharia laws which inhibit polygamy, e.g., in Pakistan:

A Pakistan court on Wednesday ruled against a man who had remarried without his first wife’s permission in a landmark verdict that was applauded by women’s right activists. ... Judicial magistrate Ali Jawwad Naqvi announced the verdict in a Lahore lower court, ordering the man to serve a six-month jail term and pay a fine of 200,000 Pakistani rupee ($1,902.95).
_{Pakistan makes landmark ruling against man for second marriage, Reuters, 2017}

If having a co-wife is not desired, conditions on monogamous marriage can be added to the marriage contract: Can a man have up to four wives? It is also possible for the potential second wife to say "I'm not marrying you without the permission of your first wife".