The statement/question from the OP was, "I’ve heard it many times it’s a sin to be in one (interracial marriage), but have never really heard any reason or purpose why." I explained how it could be interpreted as such. I also hear some folks talk about how Jesus declares that we are all one and the same blood (paraphrasing) and as such to receive
salvation through Jesus in the manner of, for example, John 3:16. But that is salvation, not about marriage.
In Genesis 2:
24 "Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
25 And they were both naked, the man and his wife, and were not ashamed." In this case of the first man and first woman on earth would have been no need to consider segregation as the Tower of Babel had not occurred therefore we were all one race. Then Jesus reiterates this from Genesis with Matthew 19, "Have ye not read, (pointing to Genesis 2:24) that he which made
them at the beginning made them male and female,
5 And said, For this cause shall a man leave father and mother, and shall cleave to his wife: and they twain shall be one flesh? There is man and woman distinguished, but no mention of race or mixed race marriage here. At this time there HAD the Tower of Babel and there were different races yet nothing is said about it and probably wouldn't have been as in Matthew they are trying to trap Jesus.
But the most compelling is with Peter in Acts 10:10 "...he (Simon Peter) fell into a trance
11 and saw the heavens opened and something like a great sheet descending, being let down by its four corners upon the earth.
12 In it were all kinds of animals and reptiles and birds of the air.
13 And there came a voice to him: “Rise, Peter; kill and eat.”
14 But Peter said, “By no means, Lord; for I have never eaten anything that is common or unclean.”
15 And the voice came to him again a second time,
“What God has made clean, do not call common.”
16 This happened three times, and the thing was taken up at once to heaven. (You can read the rest for yourself but picking up in verse 25)
25 When Peter entered, Cornelius met him and fell down at his feet and worshiped him.
26 But Peter lifted him up, saying, “Stand up; I too am a man.”
27 And as he talked with him, he went in and found many persons gathered.
28 And he said to them, “You yourselves know how unlawful it is for a Jew to associate with or to visit anyone of another nation,
but God has shown me that I should not call any person common or unclean.
As far as I am concerned, in the absence of any specific, concrete statement concerning interracial marriage, it is made clear in this account that the message and compassion and love of God is for all of mankind who calls Jesus Lord. So if a man and woman are of like mind in loving God as Father, Son, and Holy Spirit, they have been found deserving in God's eyes. It is not for me to say that their marriage is a sin. Only if they are not of the same spiritual mind and unable to follow the Lord as intended would I venture to say anything about it is a sin.