I have heard that to be member of the Sanhedrin you had to be married and therefore, Paul was a married man. Is this true?
It is true that to be member of the Sanhedrin one had to be married. The problem is that people merely assume that Paul was a member of the Sanhedrin, but the Bible never makes that statement. He was certainly an apprentice for one of the members of the Sanhedrin and therefore had the clothing at the time of the stoning of Stephen (Acts 7:58), but he himself is never said to be a member of the Sanhedrin. Of all the various claims Paul made about his Jewish identity, he never once said he was a full member of the Sanhedrin. That is why this cannot be used as evidence that Paul was married. In I Corinthians 7 he makes it clear that he was not married and there is no implication that he ever formally married. It is best to take Paul at his word that he was a single man and he remained single for his whole life.