Blogger Ched Spellman writes the following:
Could there be something more intimate than this [marriage between a man and a woman]? As it turns out, there is. Paul exhorts his readers in 1 Cor 6:16 not to indulge in sexual immorality, because the one who dabbles in prostitution become “one body” with the prostitute, referencing the Gen 2:24 text. One of Paul’s reasons for fleeing from immorality then is because “the one who joins himself to the Lord is one spirit with Him” (1 Cor 6:17). When you are joined to another human being (i.e. through marriage), you become “one flesh.” However, when you are joined to Christ (i.e. through salvation), incomprehensible as it may seem, you become “one spirit” with him!
The entire post is excellent! Read it here.