"Two men are smiling, because they are now married."
Translation:Kaksi miestä hymyilee, koska he ovat nyt naimisissa.
It's quite simply because it refers to several people. This overrides the grammatical singularity in English as well. Consider, for instance, the sentence "the couple is smiling because they are now married". Even though "couple" is singular and prompts a singular verb (except maybe not in UK English), the pronoun that refers back to it is still plural. It works the same way in Finnish.
I suppose another way to put it is that there should be subject-verb concord about number within the clause, but it doesn't have to extend to other clauses if the subject in question refers to several things.