There are some laws that are from the sages [called "from the words of the scribes"] but not from the Torah itself. For most of these laws, the reason for them is known. [But for some of them there is some doubt. E.g. the laws of Mukza on Shabat. There is an argument as to the reason.]
In any case, most rishonim [mediaeval sages] hold when the reason for a law from the words of the sages no longer applies then the law itself is automatically null and void. [This is brought in Tosphot and in the Raavad in laws of Mamrim. There the discussion is about the first fruits that have to be brought to Jerusalem. The Raavad holds since the reason for that law no longer applies so the law no longer applies.
You could apply this to marriage also. There is an argument if one is forbidden to marry someone from the Seven Canaanite nations or from all nations of idolatry. Since we do not hold from R Shimon Ben Yochai against the sages, so לא דרשינן טעמא דקרא so only the seven nations are forbidden to the opinion of the Tur.
