How to analyse the following joke?

To some--marriage is a word...to others--a sentence.

Comment:

(i) To some marriage is a word...

(ii) To some [marriage] is a sentence.

In the first clause "marriage" is object language, a mention, and "is a word" is metalanguage describing it.

The second clause has to be a use: some people feel imprisoned or punished by marriage.

The joke depends upon the ambiguity of sentence, which represents two lexemes:

SENTENCE1, 'punishment handed down by a court' or

SENTENCE2, 'a linguistic unit comprising one or more clauses'.

Since sentence in this second meaning is a metalingual term, and "sentence" occurs after "word", which can only have a metalingual meaning, we first access the metalingual SENTENCE2 and are then forced to reject it.

Goatly, A. (2012). Meaning and Humour: Key topics in semantics and pragmatics. Cambridge: Cambridge University Press.