Nope. A pirate style marriage. A marriage with no official ceremony.
I don't think there is a concept of traditional marriage between pirates in OP, correct me if I'm wrong, I don't remember it ever happening. Maybe because the concept of traditional marriage is antithesis to freedom according to Oda, so it's a big no for pirates.
The closest example we have to a so called marriage for pirates is Rayleigh and Shaqqy and that's just because they live with each other.
So that's how I imagine Sanji and Nami ending up with each other. Nami traveling the world to complete her map, while Sanji accompanying her around.