This is a polite way of saying he did not have sex with her. Alternate translation: "Abimelech had not slept with Sarah" or "Abimelech had not touched Sarah" (See: [[rc://en/ta/man/translate/figs-euphemism]])
Here "nation" stands for the people. Abimelech is worried that God will punish not only him, but his people also. Alternate translation: "even a people who are innocent" (See: [[rc://en/ta/man/translate/figs-metonymy]])