He can have opinions, but he can't have opinions on facts, and the fact is, Dean demonstrated, on screen, in more than one episode, in actions and in words, that he was on board with Sam and Eileen as a couple. To say Dean wouldn't want him with her is just not valid in the canon of the show. As to whether Dean wouldn't want to see him marry Ruby? I mean, wtf even is that? It's so ridiculous I have to wonder if he was misquoted, or maybe it was said as a 'joke' after mentioning Eileen, and the writer doesn't know anything about the character? Cuz yeah, human/demon weddings are not really a thing.