On the FAIRMORMON page that has to do with this, they write,
In response to a letter "received at the office of the First Presidency of the Church of Jesus Christ of Latter-day Saints" in 1912, Charles W. Penrose of the First Presidency wrote:
Question 14: Do you believe that the President of the Church, when speaking to the Church in his official capacity is infallible?
Answer: We do not believe in the infallibility of man. When God reveals anything it is truth, and truth is infallible. No President of the Church has claimed infallibility.
This is simply lawyer talk. Since the "President of the Church" claims to speak for God, then one must take everything he says as God's words when he is speaking in an official capacity. No one is claiming that pillow talk between a prophet and his wife is scripture. This argument is so ridiculous.
Here is how Marion G. Romney put it:
“What we get out of general conference is a build-up of our spirits as we listen to those particular principles and practices of the gospel which the Lord inspires the present leadership of the Church to bring to our attention at the time. He knows why he inspired Brother Joseph F. Merrill to give the talk he just gave. He knows why he inspired the other brethren who have talked in this conference to say what they have said. It is our high privilege to hear, through these men, what the Lord would say if he were here.If we do not agree with what they say, it is because we are out of harmony with the Spirit of the Lord.” (Marion G. Romney, Conference Report, October 1950, p.126)