I was talking with my friend the other day and while she's not a batcest shipper, she asked me why I was. I said that I don't think I take it to the same extremes some other people do, a few things that aren't a personal favorite of mine would be any of the REALLY younger kids with Bruce or say Dick and Damian. It's mostly the age gap that gets to me.
But she asked why I liked JayTim and I had a time explaining it to her and I wanted to bring it here as a sort of explanation, I suppose for people who might not enjoy batcest ships.
first of all, the name. Batcest is... an interesting name. A lot of arguments you'll come across is that they're not actually related, or there are some fanfics out there for aus where one of them (most commonly Tim or Jason) were never apart of the Batfamily in the first place. Of course, there are some where they are! Because, at the end of the day, in Canon they do share a family. But the implication of it being incest can be a real no no for people, even if it's not. Dick wasn't adopted into the family for quiet some time (I think he was adopted around the same time Tim was but I'm not a 100% sure, not to mention he acted as his ward and then aged out of it), Jason was adopted but after his return to life has regularly denied his part in the family (Look at literally any Red Hood Comic, though I do prefer to look at the New 52 Red Hood and The Outlaws for his feelings towards Bruce and the Wayne's in general), Tim was adopted but was later emancipated (in Red Robin), and Damian has a blood relation to Bruce only. Morally, there can be a problem with it but for mow shippers, since this is fiction, the lack of blood relation means there's no problem. Remember! They're not real people. Adoptive relationships, or pseudo familial ones, they don't see these relations as any reason to not ship them! And in later points, I go into why. And if people really had a problem with this pseudo familiar relationships, take for example Tim and Jason who have never once lives under the same roof nor have any legal connection to each other through Bruce, then on principal alone they should have a problem with Cassandra and Stephanie - Cass is adopted by Bruce, yes, but Bruce has also claimed Stephanie as part of the family and includes her in family talks. Does that not make them family members? No. They're friends or girlfriends, depending on how you want to interpret them.
Secondly, the canon relationships that any of the Bats have with their partners. I could go through all of them and explain the nuance of the relationships but I'll simplify for the sake of the post. In Bruce's sake, whether it be Talia or Selina or for the more fanon ones like Clark and Min Khoa (i think that's how you spell that, I'm not sure though!), there's no denying that in Canon these relationships don't pan out. Talia and Bruce have moral problems, even if she was originally a spy and didn't kill, and Selina and Bruce have an entire history of problems. For Dick, himself and Barbara are much of the same. A sweet beginning until something comes in to ruin it and they break up. Take, for example, the newest Nightwing run (on webtoon and not technically canon, i know). Barbara breaks up with Dick because she feels disconnected from him and like he's still preforming for her, but they have no conversation about this prior to it. With Kori, I'm not as knowledgeable about why they broke up, all I know is that they did and it was ugly. Another example is Tim! Himself and Stephanie didn't work out because she couldn't trust him, due to now knowing his identity, and then she faked her death, causing the distrust between them both. These are only a few examples of how relationships work out for Bats and in my personal opinion I think it's because of a ack of understanding for the characters, their love interests I mean. And in fanon, partners for Batfamily members tend to be washed down - Kon acts as an emotional crutch for Tim, Roy has morals he simply does not stand with anymore in Canon in JayRoy fics, Barbara is an anecdote in DickBarbara fics, and Selina is barely featured in Bruce fics. Not to mention the simple lack of wlw fics in the fandom that aren't background.
Due to this, the shipping of Batfamily members together is only natural. The characters share similar morals or in the case of Tim have such complicated morals that in fandom you can pick and choose what to use. They're fleshed out, have a canon relationship with multiple layers to it, and complete each other in many ways. In comparison to some of the fandom favorite ships like SuperBat or BirdFlash, where we only see a two dimensional aspect of their relationship recently, then the shipping of characters like Jason and Tim or Tim and Dick together is a natural conclusion to reach.
And Finally, History and old characteristics. It was mentioned before in another post that I'll try to hunt down for you guys and link in a repost because it will be WORLDS better than what I could write and have much more research thrown into it than this does, but originally there were a lot of implications under their relationships, specifically Bruce and Dick. Wardships programs were used by a lot of gay people when gay marriage was illegal, so were adult adoptions. It was to have a legal connection in whatever way possible. And I think in a really old comic, Dick has a dream where he's living in the manor as an adult still with Bruce. Bruce has a portrait of them together. They eat breakfast together in their pajamas or underwear, and spend all their time together. Batman regularly saves Robin over civilians and for his relationships, sometimes they wouldn't work out simply because of Dick. It worked this way for many Superheroes and their sidekicks in the "golden age" of comics! Batman has always had queer undertones since his creation but adding Robin into the mix just proved this. Like it or not, Bruce and Dick had sexual and romantic implications to their relationship since the character was first introduced in the 40s.
As for Old characteristics, in the New 52 i feel as though some of the characters have been washed down do their base characteristics. They lack depth in a way some characters don't or in a way some do, particularly in fanon. Dick's possessive originally and I see people talking about his anger issues as a child a lot, especially in comparison to Jason's Robin run, but either way - the characters have changed throughout the years. That doesn't mean that people can't cling on to their old relationships or even their old personalities. People are so fond of making Jason insane and crazy and Hella mean when in recent adaptations, he's a normal vigilante for all intents and purposes. He's not even a crime lord anymore, I'm pretty sure he still owns the Iceberg Lounge. That's all to say that the boys used to be a lot closer than they are now, especially Tim and Dick and Dick and Bruce.
The conclusion to my explanation to her is that batcest is much of the same as any other ships - picking and choosing what to take from canon to put two people together into a relationship that you like. It's a work of fiction and while that's important to remember, it doesn't say anything about you in real life! you don't enjoy real life incest if you like JayTim. That's all to say si that there's reasons behind it that some people like and some people don't. She wasn't swayed by my answer but accepted it since she couldn't deny that the Batfamily has been explored on and their relationships together so much more than their relationships with anyone else, canon or fanon.