Would it be appropriate if a Marine was the groom at a wedding and he wore his alpha dress blues?

Yes it is allowed to be worn as long as you are still in the Marine Corps

You need to move this to the Military section. The Marines have rules about wearing the uniform. If wearing that uniform to his wedding is permitted by the rules, then it is more than likely appropriate.

Sure. A friend of mine wore his Air Force Dress Blues for his ceremony when he got married. His fiancé requested that. One might request military guests (at least males) wear their dress uniforms, even make it a partial or full military wedding.

Aren't you supposed to?

i dont see why not, its his wedding and he can wear whatever he wants