By the way I wasn't objecting to Littlefinger's death in the slightest. Slimy worm got what was coming to him. But Starks are supposed to be honourable and to do things properly. Sansa only just said a few lines ago "It's not what I want, it's what honor demands." She would have known that the correct form would have been to say "In the name of Jon Snow, King of the North: I, Sansa Stark, Lady of Winterfell, sentence you to die" (that being a nice reflection of Ned Stark's line in s1e01) - THEN Arya cuts his throat with the knife he used to cause so much of this grief (no problem with Sansa not following the rule of "the one who passes the sentence should swing the sword", Arya being so much better at killing). Honour is then satisfied.
Rhaegar decides to ditch his wife - with no grounds whatsoever for an annulment.
Didn't someone say up thread that Rhaegar's previous wife (Elia Martell) had secondary infertility? Would that have been grounds for an annulment? (Not the "reason", but the grounds within the law)
Also it's quite possible that the wedding took place when Lyanna was already quite pregnant with Jon. The order of events could have been:
Lyanna (consensually) and Rhaegar start having lots of shagging in secret. Lyanna gets pregnant.
When she realises she's pregnant - which could be not for a few months in the absence of ClearBlue tests - she realises there's going to be some trouble given that she is betrothed to Robert so they run off together.
Robert kicks off Robert's rebellion and starts killing people.
After a lot of fighting which could have been over a number of months, Rhaegar realises that his family are going to lose the war and he gets his marriage to Elia annulled at that point and marries Lyanna - possibly hoping this will allow him to get Ned Stark onto his side and get things to turn in his favour. (though we see Lyanna in profile and there's no huge baby bump, she could be 5 months along and still not look very pg)
However then the sack of Kings Landing and the murder of his children by Elia Martell happens - we know that they must be already dead by the time "Jon Snow" is born.
So I guess my theory is that the whole annulment and marriage to Lyanna was a strategy from Rhaegar to diffuse the hostility against him and turn the tide of a war already raging, rather than having been his plan all along? Obviously it didn't work, and by the time the baby was born the tide was so far gone against the Targs that actually it would have endangered the Starks to have the knowledge public.