Thanks for all the replies.
However, it still leaves the original question unanswered.
If you are on the birth certificate but not married when the child is born, then marry afterwards do you get the same rights as you would get if you were married when the child was born, or do you need to either go to court or wait until they are 7 and do it at the local amphur.
This guy seems to imply that you do get full rights by marrying (the last sentence of the video), but I can't see where on the internet that backs this up:
https://www.facebook.com/watch/?extid=MSG-UNK-UNK-UNK-COM_GK0T-GK1C&v=395069283668798
There are a few people I know that are living with their girlfriend and kids but are not married (including myself), it would be interesting for them (and me) to know what the actual law is on this is.
Note this is not a visa related question, if you are living with the child and the mother and on the birth certificate immigration will give you a Non 0 extension.
This is about the legal status of guardianship and what happens if the marriage then goes South and you divorce - are you then the father in the eyes of the law or do you still need to go to court.
An example of how this might affect somebody in real life is putting a property in the child's name, if things went south and the relationship with the mother broke down after the remarry, if you were not registered properly as the father she could sell the property and you wouldn't be able to stop it, as far as I know.
Or applying for a passport for another country for the child would require documentation saying you are the legal father, but would you have it if you married after the child was born...?