We say that the Bnei Israel recieved the Torah Beyirha at Har Sinai and at purim : Kibelu Behava Ma Shekiblu Beyirha. It seems as though we hadn’t the choice! How can we reproach to the goyim not to keep the sheva mitzvot since they’ve never been obliged like we were with the Torah...they have a great argument there!

Those gentiles who don't keep the seven Noachide commandments- who kill or steal or commit adultery etc,- cannot possibly have a great argument that can justify their immorality. The seven Noachide commandments- accepted by Noah and passed down to his sons- are basic to ethical human behavior.