Very thankful for this thread. It has really set me to studying this passage and praying about it a lot. Though I have only been praying for insight, that freshness of insight from the Spirit is bound to send me to prayer and consideration for my own application of this passage. I can assure you, this passage has everything to do with the greater context of the letter.
I have been studying Paul's letters for a long time now, though I am not a theologian and I am far from qualified, experience has shown me time and time again that Paul is not "proverbial" in his letters. What I mean by that is oftentimes he appears to be digressing and saying something that seems like its own little proverb or thought. I think we need to not see these "digressions" this way. Paul had a train of thought that is sometimes hard to follow, but if you manage to stay afloat of the context and join the more "proverbial" seeming thought into the flow of the letter, you will find brighter light.
I was doing some research on theological exegetical articles tonight, and it turns out this is a hotly contested passage of scripture in terms of trying to define and understand exactly what Paul is really saying and what he is not.
"It is the glory of kings to search out a matter."
I honestly think if we can our traditional views of this, we may find this applies to each of us more than we had thought.
One thing is for sure for myself as I have been thinking through this, I do not want to discover at the end of my life that I was an "obstacle" to others (6:3) by joining my attempts at the ministry of reconciliation by eking Christ out of my heart because I still have narrow affections for unbelievers above Christ and his Children, for idolatrous thoughts and philosophies, or to make accord with Belial by thinking too much like a man as Peter did, and Jesus told him, "Get thee behind me Satan." Which is to say, at least somewhat tongue and cheek in lieu of this conversation, "don't be yoked with me," at least the Satan in you.
As a final thought, it is so easy to say, "I don't have a pagan business partner, I am not married to an unbeliever, guess I can move on and ignore the passage..."
Rather, the proper Christian attitude toward every exhortation in scripture is to place ourselves "under the light." Be examined under the Spirit's microscope! Is it possible that we too are "restricted in our affections?" I certainly know that I am! My affections for Christ could never increase enough. So my heart toward this is not to say, it does not apply, it is to say, "Jesus widen my affections! Show me where the blockage is, I am not effective enough! PLEASE!"