(can someone remove the flags on this page, it looks as if they are in error)
I suspect it was very common in a time when divorce was extremely expensive and even then involved proving adultery by the woman or adultery plus another factor such as cruelty by the man. People simply lived together as if they were married.
This man wasn't married to his second partner. His wife was still alive. He behaved as if he was married to her and their children were baptised and registered in his name. They appeared on the census as married and she was buried as his wife.
As Michelle also suggested, when men were transported no-one knew if they were alive or dead.
Sarah Satchell, was left with young children when her husband was transported in 1841. She was living with another man in 1851 and had a daughter born in 1843. Although she hadn't taken her partner's name, she was marked as married to him by the enumerator; maybe a case of local sympathy? . In 1860 she married another man calling herself a widow. Her husband was still alive in Tasmania. She married in Birmingham, some distance from where she had lived with her husband .I doubt anyone would have known about him.