Yuehan wrote "I've heard this said before, but Scripture does not explicitly affirm that Paul was a member of the Sanhedrin."
Scripture does confirm that Paul is a member of the Sanhedrin.
The Sanhedrin consisted of 71 judges. God thus commanded Moses, "Gather to Me 70 men of the elders of Israel... and bring them to the Tent of Meeting, so that they should stand there with you" (Numbers 11:16). This was the first Sanhedrin. Counting Moses himself, it consisted of 71 members. Since the membership of the Sanhedrin is fixed by the Torah, its number cannot be changed.
At the stoning of Stephen (Acts 7) , it was the Sanhedrin where Stephen presented his defense. Only members and the accused were present. When the decision was made to execute Stephen, Acts states that Saul (Paul) gave his approval of the execution. That was the act of an official member of the Sanhedrin. That is the reason Paul is believed to be a member of the Sanhedrin.
As for the requirement of having a wife, The Torah required all members of the Sanhedrin must be without any genetic defect including their genitals, thus it became traditional that members be married, and preferrably with a child. The Jews did not take any chances in failing the letter of the Law, they made sure they followed God's instructions in this matter very accurately.
If Paul was married, what became of his wife?
Paul may have answered this in 1 Corinthians 7 12-16
12 "To the rest I say (I, not the Lord) that if any brother has a wife who is an unbeliever, and she consents to live with him, he should not divorce her. If any woman has a husband who is an unbeliever, and he consents to live with her, she should not divorce him. For the unbelieving husband is made holy because of his wife, and the unbelieving wife is made holy because of her husband.Otherwise your children would be unclean, but as it is, they are holy. But if the unbelieving partner separates, let it be so. In such cases the brother or sister is not enslaved. God has called you to peace. For how do you know, wife, whether you will save your husband? Or how do you know, husband, whether you will save your wife?"
Paul's wife, if he truly had a wife, had the right to remain Jewish, and therefore separate from him to remain in her Jewish heritage and family. Paul was an ardent enforcer of the Jewish religion condemning Jewish Christians to death for Heresy. Suddenly, Paul meets Jesus on the road to Damascus, and he abruptly becomes a Christian far from home. It would not be unreasonable that she may have decided to seperate from Paul.