I would like you to explain the following verse:

Prohibited to you [for marriage] are your mothers, daughters, sisters, father’s sisters, mother’s sisters; brother’s daughters, sister’s daughters, foster-mothers, foster-sisters, your wives’ mothers, your step-daughters under your guardianship born of your wives to whom you have gone in, – no prohibition if you have not gone in – [those who have been] wives of your sons proceeding from your loins and two sisters in wedlock at the same time, except for what is past; for Allah is Oft-forgiving, Most Merciful (4:23)

In this verse, I specifically want to know the clarification and wisdom behind the following saying: ‘no prohibition if you have not gone in’. Please educate me if this option of marriage has any Shān-i-Nuzūl (occasion of revelation).

I think your questions can be answered properly if the verse is translated like this:

[Forbidden to you] are those ladies born of your wives with whom you have had sexual intercourse. And if you haven’t had sexual intercourse with them, then there is no prohibition. (4:23)

It is apparent that the verse alludes to the daughters of a lady who enters into a new matrimonial contract. It says that if her new marriage has been consummated, her daughters would become unlawful for her husband. In other words, her husband cannot marry any of her daughters from her previous marriage(s) after divorcing her, if he has established a conjugal contact with her.

The implication evidently is that these daughters of the lady have become like daughters to her husband because he has gone to the field whereof his wife’s daughters have been born. A marriage though is established by signing a matrimonial contract, it is the consummation of marriage that is actually the culmination, which makes the couple wife and husband in the truest sense. This now means that the wife’s relationships become those of the husbands and vice versa. As an obvious corollary, this situation does not arise if the husband divorces his wife before establishing conjugal contact with her. It is in this case, that the husband has been allowed to marry the daughter of his ex-wife, born of her previous marriage. 

I am afraid I have not been able to trace any Shān-i-Nuzūl mentioned regarding this verse. And I opine that there is, in fact, no need to find it out. It is actually the context of the Holy Qur’ān, which portrays the circumstances that must have existed when any verse was revealed.