The only crossover pairings that are offensive to me are anything involving adults with kids.
The crossover pairings that do slightly annoy me are those that pair Sailor Mercury, Sailor Venus, Sailor Mars, and Sailor Jupiter with four other characters of corresponding archetypes.
An example would be a Teenage Mutant Ninja Turtles x Sailor Moon crossover using the following pairings:
Raphael x Rei - Because they're both "fiery"
Donatello x Ami - Because they're both "smart"
Michelangelo x Minako - Because they're both "silly"
Leonardo x Makoto - Because they're both "protective"
In TMNT the latter pair does get swapped with MikeyxMako and LeoxMina since Venus is the leader and could go with Leo who is also the leader (LeaderShip), and Mikey likes to eat while Mako likes to cook...but the point still stands. It doesn't have to be TMNT. Any fiction where there is a "big four" and the Sailor Guardians interact with them at least one gets paired with the identical counterpart. I can understand why it's done, but it seems as if "like repels like" and "opposites attract" are overlooked, which is weird because Sailor Moon's central romance is an example of the latter.
But what if it's half-siblings and accidental and innocent and they're all cute and friendly together and genuinely love each other and they don't find out they were related until they've already been married and have a kid and by then it's all sad and awkward and tragic but they still love each other?!
...hypothetically. For no particular reason. Definitely not writing anything weird.
Also, I have decided that Jadeite / Pit is the most offensive crossover pairing.
But seriously, I can't get mad over weird pairings. Even pairings that are uncomfortable in-story. Like, I personally don't like Diamande/Usagi at all, and I don't like Chibi-Usa/Mamoru either, but I can't fault people who put their own spin on it and come up with some story where there's a little more depth to it.
As far as franchise mixing, though, which I think is the main question of the thread, I don't really mind people doing that, I just have a hard time seeing most of them myself. Like, Vegeta/Rei I can definitely see why people come up with that but I don't really buy it? Or what about Naru Narusegawa / Gurio Umino, eh? Eh?? It would be just like Love Hina but with swirly glasses. Seta Noriyasu can be replaced by Nephrite and his van with his Ferrari, which he is now inexplicably just as prone to crashing.Dang it, now I want to see Love Hina but with all the characters replaced by Sailor Moon characters.. Someone in an old thread suggested Harry Potter / Hotaru; that might be genuinely cute if it were done well enough (or dumb if it wasn't).
Basically, I have no problem with almost any ship, in principle.