Why is the reward for a woman who has preserved her chastity that her husband will marry seventy of the houris in Paradise, and is this injustice? And why has Allah not abolished gender in Paradise?

It is Allah Almighty who has prepared seventy houris for men in Paradise. Allah is not to be questioned about what He does, due to His knowledge and wisdom. All of Paradise is a bounty from Allah, and no one has a right over Allah. Had He not given His servants any blessings, He would not have been unjust to them. Injustice is depriving a person of his right. One should not delve into these matters; rather, one should focus their efforts on what benefits a Muslim in their religion and their worldly life.