Reminder that Paul is one to talk, given his life long obsession to be as much as Peter III as possible because Catherine heavily implied that Paul was not, in fact, Peter's biological son but Saltykov's. And of course Alexander himself according to the German wiki entry on his sister-in-law was rumored to have fathered one of her kids.

...I think the lone happily married Czar and Czarina in several centuries must have been the last ones, Nikolas and Alexandra. Who absolutely sucked as monarchs but were very much in love with each other till the end.