Sins that are bound remain unforgiven. The loosed are forgiven. The context also includes, however, the whole realm of relationships. The source of a powerful prayer is two agreeing, and the gathering of two or three in Christ's name guarantees an even broader relationship by the inclusion of the Lord Himself. People are bound into relationships by various things.
Blood relationships can be good or bad, but they are there nonetheless.
Business relationships are usually bound by contract, but the paper is only worth the profit it is written on. He wants us to agree on what to work for and pray for. He wants us to gather, and to gather in His name, with Him right there among us. That brings us to the procedure often associated with church discipline. In the sinful world, if one person wrongs another, the result will be retaliation.