you ask if he may just have run off or if couples would separate without divorce. At that time, divorce was only by act of parliament so only for the (very) wealthy.
https://www.parliament.uk/about/living-heritage/transformingsociety/private-lives/relationships/overview/divorce/
So, yes, you do find couples separating and remarrying bigamously. But if the person was known locally, someone might object to the banns. On Alice's marriage, which was after banns, there is no mention of her being a spinster or widow. The curate conducting the marriages doesn't seem to note marital status on the marriages and the witnesses look to be regulars as their names are on other marriages.
osprey, many thanks.
This reply is addressed to your kind self and everyone else who's contributed here.
I've used all the suggestions which have been made, and they opened a door for me, and I've discovered what really happened in 1819/20. One more story to add to the pile: another example of human behaviour, and it may help someone else to untangle their own puzzle.
In this particular case:
2 baptisms, 3 months apart, and I asked why?
Moreover I couldn't find a death for the father and I wondered if he had run off, or if the couple had separated and remarried 'bigamously', as you (osprey) say. Well, yes, they had.
The father of the 2 sons (now aged about 30) clearly had a relationship in 1819 with an under-age girl while married to the mother of the 2 sons whose baptisms I was enquiring about. He remarried in autumn 1819, to a teenager who was 6 months pregnant (it needed her father's permission).
Meanwhile his first wife was also heavily pregnant, and I'll choose to think that she threw him out! (rather than him dumping her).
But now, without husband, the first wife needed to establish herself strongly within the new land-based community (where she stayed for 40 years). She gave birth to her son on land, probably very late 1819, and booked a baptism quickly, for early Jan 1820, which was carried out.
She also needed to baptise the older son who was Born At Sea, however in early 1820 he was already nearly 7 and in good health so there was no urgent rush. So she requested a baptism as close to his birthday as possible (the day before, which was a Sunday), to make it a memorable 'event'.
The husband who betrayed her was (forgive me) very "up himself", self-important, arrogant, worked in Customs & Excise. He and his new teenage wife then produced a torrent of children with extravagant names, & he moved them all to London to increase his perceived status further, and had even more kids.
(I like the next bit.)
Then his fortunes changed and he became 'destitute' (see London workhouse registers) and it looks like he lost everything.
Meanwhile the wife he cheated on remarried in 1821, had 4 more children with a stable husband, and one of those children built up a fine business and became the equivalent of a millionaire, and the money passed on down.
Karma, justice, whatever. It pleases me!!
So thank you RootsChat friends for guiding me along. I'm delighted to have unblocked this block, and I'm really grateful.