Son un matrimonio sounds right to me and it doesn't sound as if you were trying to emphasize anything. You do this with all sorts of groups:
- Son un equipo de profesionales.
- Son un grupo de aventureros.
- Son una pareja de amigos.
etc. Son matrimonio without the indefinite article is not something I hear often, but that might be a regional thing. I do hear and read son pareja ("they are a couple").
It seems to me that son matrimonio and son pareja are idiomatic phrases, maybe because they refer to a very specific relationship/institution. They just don't follow the usual rules.
Note that you can say Jorge y María son matrimonio but you cannot say any of the following:
- *Jorge y María son matrimonio exitoso.
- *Jorge y María son matrimonio de artistas.
- *Jorge y María son matrimonio que ha pasado por muchas aventuras.
That is, you cannot add any modifiers to the noun matrimonio. This seems to show that ser matrimonio is indeed a fixed idiom and not a regular construction. (The examples above do work perfectly if you add the indefinite article.) In fact ser matrimonio acts like a stative predicate, while the regular version with the indefinite article is a common nominal predicate.
Son matrimonio. = Están casados. = "They're married."
Son un matrimonio. = "They're two people married to each other."