I think that as Andrea has no official role to play in Monaco, they are free to live their story as they want. They relationship is strong - I don't remember when they started being together, but it seems to me that they have always been a couple!
Maybe they will only marry when they want children, maybe they will never marry and have children, we don't know. I have the conviction that Andrea and Tatiana are a modern couple, that they don't feel concerned by wedding. But maybe they'll do it, for their families - as Monaco is a roman catholic state, I don't know if it would be appreciated if Prince Albert's nephew didn't follow the rules.
By the way, I agree with FanofMonaco: this present situation allows them to avoid medias attention. But I think that they are so discreet in their lives that even if they got married, medias would talk about the ceremony, and that's all. Things wouldn't change.
However I'm surprised that medias didn't sell a pregnancy story with the latter pics Iceflower posted.