I would either say, "are they getting next month?" that sounds the most natural. but you can also say "are they going to marry/get married next month?" or "will they get married next month?" I've heard people in Ireland speak this way.

are they going to be married next month ? i think it is better, but not the best question ...

"Are they getting married next month?" I assume they've organised everything so you need this grammar structure. Wenwei's examples of "going to" and "will" are certainly correct but they have different nuances.

*are they going to married next month. * can #you help me ??