Genesis 6:18

But with thee will I establish my covenant
Made with Noah at this time, though not expressed, that on his making an ark, as God directed him, and going into it at his command, he would preserve him while building it from the rage of wicked men, and save him in it and his family, when the flood should come; and that they should come safe out of it, and repeople the world, which should be no more destroyed by one; for this covenant respects that later mentioned, ( Genesis 9:11-17 ) so Aben Ezra; or the promise of the Messiah, which should spring from him, for the fulfilment of which Noah and his family were spared; and this in every article God would confirm, of which he might be assured from his power, veracity, and faithfulness, and other perfections of his: and thou shalt come into the ark;
when the covenant would begin more clearly to be established, and more plainly to be fulfilling; Noah on the one hand being obedient to the divine will, having built an ark, and entering into it; and on the other, God giving him leave, and an order to enter into it, and shutting him up in it to preserve him: thou and thy sons, and thy wife, and thy sons' wives with thee;
that is, Noah and his wife, and his sons and their wives, in all eight persons; and eight only, as the Apostle Peter observes, ( 1 Peter 3:20 ) by this it appears that Noah's three sons were married before the flood, but as yet had no children. Jarchi concludes, from the mode of expression used, that the men and women were to be separate; that they entered the ark in this manner, and continued so, the use of the marriage bed being forbidden them while in the ark.