According to the Doctor, River & River's parents (Amy & Rory), they are married. However, River & the Doctor meet in a very scrambled sequence, because they're both time travellers & their timelines aren't synchronised. This means that it's possible for there to be a meeting between them which, in the Doctor's timeline, is before their marriage but, in River's timeline, is after it. This happened in "Silence in the Library"/"Forest of the Dead". River's statement to Amy at the end of "The Wedding of River Song" that she (River) had "just climbed out of the Byzantium" shows that the same thing happened in "The Time of Angels"/"Flesh and Stone", too. It's equally possible for there to be meetings when the situation is reversed -- before the wedding for River but after it for the Doctor. We usually see events in the order that the Doctor experiences them, so it's difficult (nearly impossible) for us to know which of their meetings are in this (Doctor already married but River not yet married) reversed situation.
There have been some meetings where River has been present more than once -- "The Impossible Astronaut", for example. In that episode, the Doctor was present more than once, too. For the Doctor (both instances), that was before the wedding. For the River in the spacesuit, it was shortly before the wedding. For the River watching the shooting beside Amy, Rory & Canton, it was after the wedding.
The same lack of synchronisation between their timelines makes it meaningless to ask where River is when we can't see her -- because we've no way of knowing what point in her timeline we're asking about.
If all this seems horribly complicated, that's because it is horribly complicated.