The question for study is why do so many translations alter the passive grammar and translate the word as meaning joins himself.
Look at the context of these verses. What do they tell us?
Paul uses the same phrasing:
he who is joined to a harlot is one body with her
he who is joined to the Lord is one spirit with Him
The grammatical construction allows for either a passive or middle understanding of the participle so we must look to context for an answer.
The context is focused on the volition of those involved. A middle voice. Are they forced to go into a harlot {passive} NO. Are they drawn to the harlot {middle} YES. It is the same with Christ we are not forced, we are drawn to Him. Only God saves but He only saves the willing.
They were not forced {passive} to do either of these things. We see this view carried out in the next verse where they are to actively reject sin.
Thanks for the well stated argument for joined himself. The false doctrine in view is that if we put our faith in Christ, we join ourselves spiritually to Christ. So if that is the bogus contextual view, then the "joined himself" interpretation seems to justify changing the grammar to middle voice.
If you look back to 1 Corinthians 6:15, you see that Paul is acting upon a member of Christ's body and joining it to a harlot. Thus the passive voice in verse 16 and 17.