Eldest son/child has the same name as her own father.
He may be the father of the oldest children if not all of them. It was quite common for an older female child to, er, well take on the mother's 'marital duties' as it was seen as something a man needed.
Another possibility is religion, as someone mentioned not all marriages were recognised legally, right up to the 1990s people not marrying in a church that wasn't C of E had to make arrangements for a registrar to attend or had to have a civil ceremony usually the day before.
There is also the possibility that if they were different faiths, or different denominations then they may not have been able to marry / find someone to marry them, this is more likely in a small village with only one church.
He could have been someone who travelled a lot, possibly not British, marrying a foreigner could mean you lost your own citizenship as a woman.
The other options I think have been covered. One or both of them married to someone else, second family,
Actually I've been quite surprised by having several illegitimate ancestors in the late 18th and through the 19th century so perhaps it was more common than we think!
It was very common for the bride to be pregnant, it was more important that she was fertile than a virgin. There was no pension or social security, your children were your future.