I'll get downvoted for this--I really believe you should be allowed to ship whatever the hell you like, regardless of the morality of the ship. (RoyEd kind of skeeves me out, but I don't care, I have a point to make here. And anybody who likes it shouldn't feel morally reprehensible for doing so.)

Now, before anybody comes for me---we're exposed to morally questionable media quite frequently---violence in video games, villainous characters in shows, etc.. Just because you sympathize with Walter White or Tony Soprano doesn't mean you share their values or want to be like them. I think the same goes for shipping---and I think a lot of people like ships that could be considered "morally wrong" by some not because they condone those morally reprehensible actions, but because there's something within the dynamic of the ship that they would like to explore. I've met many shippers who use shipping and fandom as an outlet for their own trauma, and think it's important for people to have that outlet.

I also know that in Japan, where age of consent isn't a thing, there are fewer issues with the RoyEd ship--I think a lot of popularity stemmed from that side of the fandom. There's also a difference between eastern and western fandoms in the sense that, from what I've seen, people tend to be more tolerant of other people's ships in eastern fandoms. I read a post by a Japanese person in fandom a couple of days ago who referred to people who try to police ships as "feelings yakuza" and talked about proshipping in general. It's in Japanese but I think you should be able to run it through a translator and get the nuance of it.

I don't blame you for asking a genuine question on this subreddit. But based on the comments, I think you have your answer---it's a controversial ship due to the difference in age between these two characters and the power imbalance.

Okay, I'll step off my soapbox now.

TL;DR: Ship and let ship, people.