From 2 Corinthians 6:14 -
"Do not be mismated with unbelievers. For what partnership have righteousness and iniquity? Or what fellowship has light with darkness?" (Revised Standard Version, Catholic Edition)
"Be ye not unequally yoked together with unbelievers. For what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?" (Memorial Edition, Holy Bible, Concordance)
Based on Deuteronomy 22:10 and Leviticus 19:19, which seem to be straight Judaic law, warning of yoking different animals together to plow fields. Asses and oxen have different strides, it seems, and this will not only make it harder on the animals but on the plowmaster.
But is Paul speaking strictly or even tangentially about marriage in the above? It seems that many Baptists interpret this passage as strictly related to marriage between Christians and non-Christians. It could be interpreted to include this, I would think, but it should also warn of other relationships, and it seems probable that Paul was warning against uneven business dealings more than anything else. In 1 Corinthians 7:12-16, Paul also wrote:
"To the rest I say, no the Lord, that if any brother has a wife who is an unbeliever, and she consents to live with him, he should not divorce her. If any woman has a husband who is an unbeliever, and he consents to live with her, she should not divorce him. For the unbelieving husband is consecrated through his wife, and the unbelieving wife is consecrated through her husband. Otherwise, your children would be unclean, but as it is they are holy. But if the unbelieving partner desires to separate, let it be so; in such a case the brother or sister is not bound. For God has called us to peace. Wife how do you know whether you will save your husband? Husband, how do you know whether you will save your wife?"
While Paul admits that this is his opinion and not the Lord's, it strikes me as odd that he would include it and then later clarify this as "this is what you should do if you get in this situation, but don't get in this situation." Again, it's possible that's what he meant.
I'm open to hearing interpretations.