For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Gene Coleman
I think it is also possible that Peter was also a widower.
Chuck Huber
Paul was a Pharisee. Pharisees had to be married and with children. So wouldn't that make Paul a married man?
Jennifer Rothnie
While it's a common thought that pharisees had to be married, there is no primary source to support this statement. There is no emphasis on a religious need to marry in pre-70 A.D literature. There is only slightly more historical support for the idea that one needed to be married to be a member of the Sanhedrin, even.
Starting in the third century AD, there was a general practice for all Jewish men to marry as a religious obligation to be 'fruitful and multiply', especially Jewish leaders, but this was after the time of Paul. Even then there were exceptions.
Paul was still a young man (Acts 7:58, Acts 8:1-2), between 24 and 40. While Jewish men could marry at 18, many waited until they were much older (30-40). The unmarried state of a young pharisee, son of pharisees and educated by pharisees, would not be unusual.