To structure this clearly, I have started several threads, each with a specific purpose.
0) This thread is for general discussion, keep debate to a minimum but if it must happen it goes here. If in doubt, post here. This is the primary discussion thread.
Then I have three additional threads that exist simply to collate the scriptural evidence supporting each position. No discussion of other positions is to occur in those threads. Each should ultimately have each point made once, comprehensively but efficiently, so they are quick to read while carefully elaborating the position. To achieve this, I will delete any posts that stray off-topic, and may do some trimming if some points become too repetitive.
I see four basic answers to "When does marriage begin?", and I have a thread for each position.
1)
Sex forms a marriage (if you have sex you are married by default, unless you were ineligible to marry)
2)
Covenant forms a marriage (marriage is formed by contract / agreement / covenant / consent, sex may then occur within marriage)
3)
Possession / Either forms a marriage (if you have a woman and nobody else objects, she's your wife)
4)
Both form a marriage (only once you have a covenant, and have consummated it with sex, are you married).