This alliance is still in its inception phase. The fact that the Marines know about it is because X-Drake is a spy. Essentially, this banquet functions as the official announcement of the BM/Beast Pirates alliance. However, I dare say it will not even survive this arc and that the might of at least one Yonkou, possibly two, will be broken.
By default pirates alliances are very unstable and this counts even more for this one considering the erratic nature of BM & Kaidou and their past history together. The entire Xebec thing is nice and all and it might still play a role later on in the story, but at this point it's primarily meant to underline the magnitude of this alliance, which is unprecedented in history. How do you think the world will react if they learn it has been broken before it even began? And more importantly how they will look at the ones who were responsible for this? Even the War of the Best at Marineford would pale in comparison. The incidents at Totto Land were a first 'test'. The Strawhats provoked a Yonkou and got a way with it. Now it's the real deal and overcome them.
The idea of a marriage is to solidify the alliance, but if it doesn't survive this arc, then that's just what it was: an idea. It would not or barely come to fruition, but it doesn't need to. The whole White Cake Island arc was centered around this concept, so no need to repeat it. The idea on the other hand is enough to provide an extra layer to the story and additional character development to Sanji, Pudding and possibly Kaidou's Son. I mean there are still some loose ends in the Sanji/Pudding plot line.