I will get married next week.
I'm getting married next week..
Both constructions work fine, people generally use "Am/Is/Are getting married next week" (The second example) when talking about something that has been planned for a long time or you have thought about, and will certainly happen beyond a shadow of doubt, i.e, this future event will happen no matter what happens. You use this construction when you are 100% sure that nothing can either interrupt or delay it.
I'm getting married tomorrow no matter what happens!
My parents are arriving tomorrow.
I'm searching for jobs tomorrow
He is working next week.
I will get married next week if it doesn't rain.
My parents will arrive tomorrow if the whether is fine.
I will search for jobs tomorrow if I'm not tired.
He will work next week if he is feeling better.
As you can see, all these constructions were increased if, which implies to say one condition, something would happen if another thing happens: If it doesn't rain, If the whether is fine, If I'm not tired, If he is feeling better.
So baseed on these examples, I would certainly say that "I'm + verb ING" is placed in a high degree of sureless, i.e, only used when you are totally sure about one the occurrence of a future action.
You can also enjoy using this construction to express a future action, formed by: Subject + Will + To be + Verb -ING:
I will be doing this next week
He will be working next week
They will be traveling next week.