I would normally say "Are they ..". I don't know whether it would have been accepted.
It is not accepted at the moment (2015‐01‐11). It almost certainly should be; I doubt any native English speaker ever says “Is it a future couple?”. I have reported it.
If I end up asking for a future house while I'm in the Netherlands, I will really question my choices in life.
What does this mean? Is it something you would say when talking about two people who look like they might go out with each other soon?