Being in a relationship with a non-Christian is not a sin

That has no relevance to the discussion. Exodus 34:10-16 "10 And He said: "Behold, I make a covenant. Before all your people I will do marvels such as have not been done in all the earth, nor in any nation; and all the people among whom you are shall see the work of the Lord. For it is an awesome thing that I will do with you. 11 Observe what I command you this day. Behold, I am driving out from before you the Amorite and the Canaanite and the Hittite and the Perizzite and the Hivite and the Jebusite.12 Take heed to yourself, lest you make a covenant with the inhabitants of the land where you are going, lest it be a snare in your midst. 13 But you shall destroy their altars, break their sacred pillars, and cut down their wooden images 14 (for you shall worship no other god, for the Lord, whose name is Jealous, is a jealous God), 15 lest you make a covenant with the inhabitants of the land, and they play the harlot with their gods and make sacrifice to their gods, and one of them invites you and you eat of his sacrifice, 16 and you take of his daughters for your sons, and his daughters play the harlot with their gods and make your sons play the harlot with their gods." Deuteronomy 7:1-4 "When the Lord your God brings you into the land which you go to possess, and has cast out many nations before you, the Hittites and the Girgashites and the Amorites and the Canaanites and the Perizzites and the Hivites and the Jebusites, seven nations greater and mightier than you, 2 and when the Lord your God delivers them over to you, you shall conquer them and utterly destroy them. You shall make no covenant with them nor show mercy to them. 3 Nor shall you make marriages with them. You shall not give your daughter to their son, nor take their daughter for your son. 4 For they will turn your sons away from following Me, to serve other gods; so the anger of the Lord will be aroused against you and destroy you suddenly." These commandments were given so that Israel would not mix with other pagan nations and defile the Jewish bloodline for the Messiah, but after his coming, there was no need to make a distinction between Jew and Gentile. 1 Corinthians 7:12-16 "12 But to the rest I, not the Lord, say: If any brother has a wife who does not believe, and she is willing to live with him, let him not divorce her. 13 And a woman who has a husband who does not believe, if he is willing to live with her, let her not divorce him. 14 For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband; otherwise your children would be unclean, but now they are holy. 15 But if the unbeliever departs, let him depart; a brother or a sister is not under bondage in such cases. But God has called us to peace. 16 For how do you know, O wife, whether you will save your husband? Or how do you know, O husband, whether you will save your wife?" 1 Corinthians 7:39 "39 A wife is bound by law as long as her husband lives; but if her husband dies, she is at liberty to be married to whom she wishes, only in the Lord." The phrase "In the Lord" has a quite different meaning than what we perceive. Eph. 6:1 "1 Children, obey your parents in the Lord, for this is right." We are not to obey our parents only when we are Christians, but because of God. Another commonly quoted passage is 2 Corinthians 6:11-18 11 " O Corinthians! We have spoken openly to you, our heart is wide open. 12 You are not restricted by us, but you are restricted by your own affections. 13 Now in return for the same (I speak as to children), you also be open. 14 Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness? 15 And what accord has Christ with Belial? Or what part has a believer with an unbeliever? 16 And what agreement has the temple of God with idols? For you are the temple of the living God. As God has said: "I will dwell in them And walk among them. I will be their God, And they shall be My people." 17 Therefore "Come out from among them And be separate, says the Lord. Do not touch what is unclean, And I will receive you." 18 "I will be a Father to you, And you shall be My sons and daughters, Says the Lord Almighty." Some point this to say that dating unbelievers is a sin, but this passage is speaking of refusing to unite with unbelievers in preaching false gospels. Thus there are no scriptures used to support the notion that dating/marrying non-believers is forbidden.