Is the covenant between spouses to consider the marriage void in the event of marital infidelity permissible in Islamic law? And does the marriage actually become void upon the occurrence of infidelity?

The marriage is valid if the spouses stipulate in the contract that the occurrence of adultery by either of them grants the option to the other party. The marriage is not annulled by adultery because it is a sin that does not lead one out of Islam. The wife may stipulate what is in her interest, such as the chastity of the husband. As for the husband, his stipulation of the option is an affirmation of his right to divorce. If marital infidelity is meant to be the husband marrying another woman, then this is a suspension of divorce. If the husband intended divorce by the phrase "then the marriage is considered void" when uttering or writing it, and then marital infidelity occurred, the divorce has taken place.