The Targs were also polygamists so was a bit surprised they put the divorce in there. Always assumed he married Lyanna and was just going the sister wife route. Kind of wonder if GRRM will write the divorce.
I think he kind of has to, if he wants Jon Snow to be the legitimate heir to the Iron Throne. Remember, Bran called him "Jon Sand" before Sam informed him that Rhaegar annuled his marriage to Elia and then married Lyanna (although I think Bran is wrong - Jon wouldn't be a "Sand" as, although he was borne in Dorne, neither Rhaegar nor Lyanna are Dornish. He would be whatever the bastard name for the Crownlands would have been - although I can't look that up right now because the ASOIAF Wiki is down right now for whatever reason). I get it, the Targaryens were both incestuous and polygamists, but they still have to worry about lines of succession.