This sentence will make sense in context. As you may know, you do not always need to say the pronouns in a sentence. For instance "Leap the fence." The sentence is a demand. Who is it to? The demand is to the listener; an understood 'you.' That is what your sentence has done. It is an understood - silent you, you two, or we. We would know the answer of the pronoun in context, as we would the meaning.

It can mean both, but would likely mean the first. It would be used in a case where people might expect the person being asked to be married by now, but he or she has delayed or refused. It would be taken as "What are the reasons you aren't married yet?" It might be use by one partner to another that is reluctant to marry. The second would be rarely used. It would be taken as "We've been going together for so long, why don't we just get married." This would be the stupidest, least romantic and awful marriage proposal I could think of.