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I concur with your estimation on the opinions of Paul in that it is not necessarily a command from God.
11 But and if she depart, let her remain unmarried or be reconciled to her husband: and let not the husband put away his wife.
12 But to the rest speak I, not the Lord: (Jesus did not speak about this that we know of) If any brother hath a wife that believeth not, and she be pleased to dwell with him, let him not put her away.
This does not mean that a christian has the right to put away a wife that believeth not if she is pleased to dwell with him, using the excuse that Paul said that it was optional to follow that advice. That would be a bad interpretation of "speak I, not the Lord" He simply means he did not have a command from the Lord Jesus Christ to refer to as he did with verse 10 When Jesus specifically said But I say unto you, That whosoever shall put away his wife, savingfor the cause of fornication, causeth her to commit adultery: andwhosoever shall marry her that is divorced committeth adultery. (Matt 5:32)