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I'm thinking it has to do with the marriage of the Lamb as someone above suggested.
Here's how one commentary identifies the mystery referenced in Eph 5:31-32. (partial)
(Christ) himself is a great and unfathomable mystery:
Beyond all question, the mystery from which true godliness springs is great: He appeared in the flesh, was vindicated by the Spirit, was seen by angels, was preached among the nations, was believed on in the world, was taken up in glory. (ESV, 1 Timothy 3:16)
To add to this mystery of Christ that somehow every believer is united into a single body in his flesh, is indeed a very great mystery that no man can either fully know or explain. That believers are literally in some mystical sense united to the person of Christ and thus died and risen in him to new life is central to Pauline theology (Col 2:6-15, Eph 3).
It is amazing that we are united to the person of Christ and to every other believer the moment that we repent and receive Him in faith... for ETERNITY and that we will sit in judgement of Satan and his fallen angels (who have done all they can to discourage us, distract/tempt us from service to Christ, swell our pride, and to doubt our faith in this life) one day (1 Corinthian 6:3)
I think it circles back to Genesis. Man and woman are reunited in marriage after becoming separate when God created Eve from Adam. The mystery is Christ and the church now revealed. God designed marriage in the beginning to reflect that picture of the gospel. It was His plan all along. We were separated by sin but Jesus made us whole again.
Also note the "for this reason" at the beginning of this passage. The "this reason" it refers to is found in verses 28, 29, and 30 which immediately precede it. Paul's sentence construction leaves a LOT to be desired, so you have to remember to chase down those sentence fragments!
I read recently (in a fiction book) the reminder that we are not bodies who have souls, but souls who have bodies. If you remember that the essence of our being is the soul, and the "flesh" is just a suit, the one-flesh seems to me a lot more straightforward. It's just like having 2 sleeping bags and instead of using them separately, you are zipping them together. The church, as the body of Christ, just does that on a larger scale.