If they are going in opposite directions in time, then surely that should be River's last kiss, but it can't be as they haven't even married yet.

So how could that be her last kiss, it was clearly the Doctor's first kiss though which should mean it was her last one?

If he can't remember ever kissing her before, and she's travelling in time towards his past, it means that she'll never kiss him again (cos he hasn't kissed her in his past)

It is her last kiss. At that point I would say she is married to him (if they get married that is) while the doctor isn't. I think perhaps the reverse order is a little oversimplified anyway, and that's just the general order they're going in.

No one said they were married. But even if they were it would still work River has married the Doctor but the Doctor has not married River yet because the so called "marriage" is in the Doctors future but Rivers past... But the marriage may not even happen

They married in River's past. The kissing, the marriage, the relationship are her memories.

They will marry in the Drs. future. His first kiss, the relationship has yet to come

They're explicitly not going in exact reverse order, or they couldn't both have both Jim the Fish and Easter Island at any point — it wouldn't even be worth discussing because they'd never have any events in common whatsoever. I think Moffatt's tried to simplify a little for poetic reasons and not quite thought it through.

But I think they will kiss again won't they? In the library, River says that the future him turned up on her doorstep and took her to derillion or something. They probably kissed then I assume. But neither of them will know about this yet as it is in both of their futures coz they're not going completely in opposite directions.