Where did you get the idea that Paul was unmarried? Do you presume the same of Peter?
Paul was unmarried. Peter had a wife. Paul said in talking about sexual conduct, "I wish you all could be as I am . . . " and went on to exhort them to marry if they couldn't keep their hands off each other (my paraphrase).
It has nothing to do with protestantism or catholicism but I believe in all my heart that Paul was unmarried. I don't know about the other apostles. I suspect John the Beloved never married either.