I was literally crying, asking to marrying in an omegaverse au is just to sweet and pure. ...

Right.
Because you can marry again if it doesn't work out but becoming mates is a whole different thing.
In marriage it's "til death do us part"
In pairing it's "only in death do we part" lol
I honestly like this part. The part where once you become mates, your body will take no one but your mate's. So cheating is out of the question.