For “good” morals and “good” social behavior does not make anybody a “good” person. Christianity is not a religion of human accomplishment, but rather a relationship of divine achievement, as faith in Jesus Christ — whose blood shed for our sins in our place — is the only thing that can make us “good” or “right” in God’s sight.

All of us, without faith in the justifying work of Jesus on the cross, are in and of ourselves bad, depraved to the inner most, radically corrupt in every part of our being. But by the grace of God, Christ was crushed for our iniquities, and only thus we can be forgiven of our sin and cleansed from all unrighteousness. (cf. Romans 1-4)

Any extreme on either end will be futile. Conservative fundamentalists overemphasize to Rom 12, while liberals/emergents over emphasize 1 Cor 9. When there is a good balance, we will be reaching out without selling out our morals, values, and identity. We need to be contending for the truth and the Gospel, as well as contextualizing it so it’s relevant to unbelievers around us.

On 2 Cor 6:14 MacArthur writes,

unequally yoked together. An illustration taken from OT prohibitions to Israel regarding the work-related joining together of two different kinds of livestock (Deut 22:10). By this analogy, Paul taught that it is not right to join together in common spiritual enterprise with those who are not of the same nature (unbelievers). It is impossible under such an arrangement for things to be done to God’s glory.

with unbelievers. Christians are not to be bound together with non-Christians in any spiritual enterprise or relationship that would be detrimental to the Christian’s testimony within the body. This was especially important for the Corinthians because of the threats from the false teachers and the surrounding pagan idolatry. But this command does not mean believers should end all associations with unbelievers; that would defy the purpose for which God saved believers and left them on earth. The implausibility of such religious alliances are made clear in verses 14b-17.

(The MacArthur Bible Commentary)

Certainly, this includes marriage as it is a union of 2 people becoming one flesh — which involves mind, heart and soul — and dating relationships, which aims towards marriage. They’re relationships which would be detrimental to the believer. In all relationships, we ought to remember the Great Commandment and Great Commission, and further, to love our enemies and pray for those who persecute us.

I think the context of 2 Cor 6 was not originally intend for “marriage” per se, otherwise Paul would have mentioned it explicitly; our application of it to marriage is rightly appropriate because of the spiritual and emotional depth that is involved.