Hey!
With Dennis and Dee, it’s a very fine line, and my canon vs. non-canon distinction has always been a little arbitrary. It actually makes me really happy whenever someone disagrees with me about how to categorize their relationship, because I would love to think of D/D as canonically incestuous.
My system for deciding whether to label a ship as canon or non-canon is based entirely on what I think the creators intended to show the characters’ feelings are. (The exception to that being if the characters canonically commit incest, even if it’s not based on a canonical sexual attraction to each other. I still call that canon.)
Which means that even if something that comes off as really incestuous happens, if I don’t believe they are definitely trying to show that the characters actually have incestuous feelings for each other (or at least one of them does), then I don’t count it. So this would be something like Dennis and Dee playing love interests in The Nightman Cometh. I can see the writers and the actors going “haha this is so incestuous”, but there’s no actual concrete indication there that D/D feel anything for each other. The same thing, sort of, with the gang investigating the possibility that Dennis might have been the one to get Dee pregnant. “Dennis being so drunk that he might have even had sex with sister, haha so funny.” Because I do definitely think the writers (and of course Kaitlin) realize that some of this comes off as very incestuous, but I think they play it for laughs in such a way that the actual feelings of Dennis and Dee don’t come into it? It’s hard for me to explain the distinction I’m trying to make. It’s like they’re acting incestuous but it’s not actually based on incestuous feelings, it’s just a funny thing that’s happening. (That’s my view of the writers’ intentions, not how I view the ship itself.)
The Gang Broke Dee is a little ambiguous, as Dennis’ pouring his heart out to Dee as she’s about to leave on the plane could be interpreted as a romantic confession. But it is still up to interpretation, in my eyes, anyway. (Believe me, I wish I felt like it was written in stone romantic.)
I agree with you that Who Got Dee Pregnant? is the episode that poses the most trouble to categorizing their relationship. Many people have pointed out that it’s never stated in the episode that Dennis and Dee didn’t have sex, we only know that Dennis isn’t the father of Dee’s baby. But that’s just a headcanon, even though it’s a very convenient and easy one to have. The real issue (/gold nugget) here is that Dennis actually thinks it’s possible that he and Dee had sex. And that is almost enough for me to call it canon. Almost.
But my “canon” classification is designed for the benefit for those followers of mine who are unfamiliar with It’s Always Sunny In Philadelphia. And I feel like I have an obligation to not mislead them into thinking that there is more to Dennis/Dee than there really is. I just don’t want anyone to be disappointed, or, God forbid, start a 10+ season show only to find out that nothing ever actually happens between them. (Yet…??? PLEASE!)
Honestly, Dennis and Dee belong in a third middle category, along with a select few other ships. But I had to make things easy on myself and only do 2 categories. There’s a difference between the “canonicity” of their relationship and that of the most non-canon of non-canon ships on here. But there’s also a huge difference between them and the most canon of canon ships on here. And since those of my followers who are looking for canon are really hoping to find something super duper hardcore canon, I try to reserve my canon tag for situations that are a little concrete.
I’m sorry this is so long! The thing is, I really do sort of agree with you which is why I had to explain why I classify it the way I do. And of course I just love talking about Dennis and Dee. There is so much subtext, that even if it wasn’t intentional, it all just adds up to something.
Thanks for your question!