Is a man’s marriage to another woman—for the purpose of procreation—considered an injustice to his infertile wife, or is his not marrying considered an injustice to him?

Marrying a second wife is permissible and may be recommended for the sake of having children, as the Prophet, peace and blessings be upon him, said: "Marry the fertile and affectionate woman, for I will boast of your numbers." There is no injustice to the first wife in this, as a legal ruling. Injustice only occurs in practice when people do not deal justly between the two wives. It is not permissible for a wife to prevent her husband from marrying another. Not marrying a second wife does not constitute injustice to oneself, unless the man fears falling into sin.