Was it Paul's opinion ?

Q: How can Christians answer this? 

A: What does the verse say?

These kinds of questions are typically presented by today's God haters (incl. idolatrous false Christians; progressives) who want to dismiss Paul. But Holy Scripture, [the 66 books], is the infalliable and inerrant Word of God. It will remain as absolute truth, whether those vipers acknowledge it or ignore it. 

The answer is: "Lord" is often used specifically to refer to Jesus.  It's possible he meant that Jesus had not spoken on this specific subject during His earthly ministry, nor to Paul directly about this. It appears then the Holy Spirit was moving him to speak on that and we know since all Scripture is God breathed, it came from the Spirit. So remember, God uses people (in OT & NT) to reveal all things to us by the Spirit & that includes by speaking any on righteous principle. 

Also Remember in Matthew 16:16-17

“And Simon Peter answered and said, Thou art the Christ, the Son of the living God. And Jesus answered and said unto him, Blessed art thou, Simon Barjona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven.”

And 2 Timothy 3:16 says

"All Scripture is God-breathed and profitable for teaching, for reproof, for correction, for training in righteousness"

Also...

Paul notes, "The women are to keep silent in the churches; for they are not permitted to speak, but are to subject themselves, just as the Law also says. If they desire to learn anything, let them ask their own husbands at home; for it is improper for a woman to speak in church." 1 Corinthians 14:34-35

Notice when Paul was addressing this topic in vs 34 he added..."as also saith the law".? It was already in the old testament Scriptures before Paul talked about it. Thus it is from God. 

REMEMBER; " If any man think himself to be a prophet, or spiritual, let him acknowledge that the things that I write unto you are the commandments of the Lord". (vs 37).

And, that even when he sent Timothy (a reliable trustworthy disciple of sorts of Paul) to go straighten out churches "on his behalf", 

1 Cor 4:17 "Timothy will remind you of my ways 'which are in Christ', just as 'I' teach everywhere in every Church". Timothy presented him well and perfectly. Paul doesn't teach anything that isnt of righteousness, holiness, proper living as a believer, from all he learned [as a well educated man in Judaism (under the top religious rabbi in Israel), unlike the other Apostles], as well as specific things directly told & taught to him by Jesus, besides as noted before, all that the NT authors wrote was God breathed of and written in Scripture, and is for our learning. And Timothy became Paul's protege & successor. 

Notice What Peter said about Paul's writings. (See paragraph 2)

So we learn:

1. God directly told those in the Bible times something.

2. The Holy Spirit moved men to write something/their book(s).

3. There is more to what Scripture says than is in the textual written words in black or red in a verse. Such as specific (and general) principles, truths, and doctrines. 

4. The righteous men in the Bible God used,  understood Scripture both in word and principle. 

5. Peter said Paul's writings were Scripture.

6. What Paul said was from God. 

7. All of Scripture is breathed from God. 

(*the 66 books in the Bible)

As for those who had not understood this before or those who were decieved by false preachers, pray God opens their eyes to these truths. That you now know. 

As for the Bible haters, those idolators, pray God opens their eyes so they acknowledge, bow to God's Word, surrender (to salvation), to its truth and uphold it as such as they live in truth. 🙏✝️

Footnotes:

(1) "Now, people get into the book of 1 Corinthians, and they like to play around with some of it, and they say, "Well, Paul speaks in 1 Corinthians 7, and he says, 'I say, not the Lord...'" and they say, "'See, that's just Paul's opinion.'" Nowhere in the book of 1 Corinthians does Paul offer his opinion. This is God's word, and that's true of all the Bible." Called to be Saints

We know the Scriptures are fully God Breathed, so whatever The New Testament writers wrote, came from God. In such places  the Holy Spirit in him revealed it, it just wasnt a specific command God "told him" or something "directly told" to him by God. Think back about how even Peter spoke truth from God without God saying it "to him". In the Gospel of Matthew (Matthew 16:13-17), Jesus asks his disciples who they say he is. Peter responds, "You are the Messiah, the Son of the living God." Jesus then replies, "Blessed are you, Simon son of Jonah, for this was not revealed to you by flesh and blood, but by my Father in heaven." so we know it is in fact from God. Going back to truth revealed of Scripture, in 2 Timothy above.

(2) Same topic addressing the claims of "Paul's opinion", further clarified. 

(3) A further look at women topics  

(4) This shows Scripture itself said Paul's writings were all from God.