I am such a man, and I did choose to get married formally, of my own free will.
Furthermore, we got married in a civil ceremony, in Cyprus, not in a religious ceremony.
As to why, let's just say that marriage has a big symbolic meaning, which is not religious. Part of it is being "really" married, including the formal awcknowledgement of the state.
By the way, I just realized, while writing this, that I would have to get married formally for my wife to get her dependent visa (for the US), so there are formal reasons to get married too.

(apologies for the english, my laptop is broken)