I've been wondering this since I've seen the movie. Tifa has a ring on her finger, for one, and two, her and Cloud have a conversation sorta like this:

Tifa: You have geostigma, don't you?
(Cloud doesn't reply)
Tifa: We can fight it. Together.
Cloud: There's no cure.
Tifa: That's not stopping Denzel, is it?
(Cloud doesn't reply again)
Tifa: Or... do we have to be a REAL family for that to happen?
(Cloud looks at her)
Cloud: Tifa...
(Tifa looks over at Cloud)
Cloud: I'm not fit to help anyone. Not my family, not my friends. No one.

And when she says, "Or do we have to be a real family for that to happen?" I think it means that they're not a REAL family (like related) but they could be engaged or married. Any thoughts?