We know that Paul was not married at the time he wrote 1 Corinthians, but not of his marital status prior to that. As you say, it is logical to assume he had been married before, but we have no proof. By the same token we have no PROOF that Paul had any children. Maybe he did, maybe he didn't. It only becomes a critical point if you are going to insist that Paul meant that ONLY a man who was married and had children could be a leader in the church. That makes no more sense than the Catholics insisting that a priest should not be married because of Paul's statement in 1 Corinthians.
Since we lack the proof of Paul's marital or parental status, a reasonable reading of the scripture, to be in harmony with his writings to the Corinthians, would be that to be a bishop, if one is married, he cannot be married to more than one woman; if he has children, he needs to have raised them well, to be obedient and in submission. To read the scripture to REQUIRE a bishop be married and have children would require us to make assumptions about Paul that you yourself have pointed out cannot be proven either way.
1 Timothy 3:2 It behoves (δεῖ) therefore the overseer...
Greek - δεῖ - what must happen, i.e. what is absolutely necessary [http://biblehub.com/greek/1163.htm]
Now, you can argue and conjecture all you want about Paul's status as to having been married or not. As I mentioned, by all accounts he was Sanhedrin and they were required to be married. That aside. He still put the requirement for "overseers" what he did.