I think Dany will have a fast labor and have Lyannas fate and Jon wont have time or have to do any marriage.But I also think will defeat the nights king but be killed off from cersei plot like ned. Its gonna be a sad ending cause there child likely will never know there parents cause Martin and D and D hate a good ending.
Too me i think Dany and Jon hook up is not so grosse because 1 they dont know and b what are they suppose to do about those feelings. Its not like they were raised together. If Dany gets pregnant there is no way jon will turn his back on her and a baby. Sure Jon will need time to figure out how too handle things. But Jon is not some punk who purposely would hook up and then dump someone not after how he lost ygritte for nothing.