Mdthbs

Salam alaykum.
Please note that I myself am not in this position, rather I am in the position of knowing men who say they do "halal cheating"; namely marrying a second woman making the sexual activities with the second wife permissible in islam. I am not knowledgeable enough to do more than to advise them it is wrong to trick your first wife and not take her choice into account or her comfort.
However, what I wonder is if someone may be able to cite islamic texts to show that in islam it is actually wrong to hide such a thing from your first wife. I am sure that many of these men would, in knowledge that it is haram or makruh to lie to your first wife, stop having these thoughts.

The entire "halal cheating" seems to be a form of self-validation for people who are no better in spirit than the fornicators who have extramarital sex, as they also give in to their compelling desire for lust.

I am very grateful for anyone who would be able to provide islamic citations!

It is legal in Islam for a man to marry more than one woman.

فانكحوا ما طاب لكم من النساء مثنى وثلاث ورباع

Then marry those that please you of [other] women, two or three or four.

^{— Quran 4:3}

If the marriage was proper, e.g. in the presence of two witnesses and the wali of the bride, with proper offer and acceptance etc. then it is a valid Nikah and is incomparable with fornication or extramarital sex. There is no blame for satisfying desire, provided that it is through legal means.

Neither is the consent of the first wife required nor is the husband obliged to inform her. What he is obliged to do however is to administer justice between his wives, e.g. in division of nights between them. And he is also advised to remain with a single spouse for that purpose.

فإن خفتم ألا تعدلوا فواحدة

But if you fear that you will not be just, then marry only one

^{— Quran 4:3}

Note that although it is not legally required to seek the approval of the first wife, however doing so may be recommended as part of courtesy and good treatment:

وعاشروهن بالمعروف

And live with them in kindness.

^{— Quran 4:19}

لتسكنوا إليها وجعل بينكم مودة ورحمة

That you may find tranquillity in them; and He placed between you affection and mercy.

^{— Quran 30:21}

أكمل المؤمنين إيمانا أحسنهم خلقا، وخياركم خياركم لنسائهم خلقا

The most complete of the believers in faith is the one with the best character among them. And the best of you are those who are best to your women.

^{— Jami at-Tirmidhi}