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The Lumati. Aliens whose equivalent of a signature in a deal is having sex with the other party. Even if the other party is, you know, of a different species. Most likely a deliberate homage to the concept of "rishathra", introduced in Larry Niven's Ringworld novels.
Sheridan's uber wallbanger:
Future Londo: Make sure to clean up after your wars! Sheridan: Okey doke!
And then Sheridan FORGETS. You could argue that this is because B5 has an apparently immutable timeline, but Sheridan doesn't seem to do a thing to try to stop it, and it's totally forgotten outside of this meeting.
He knows what's coming and then acts surprised when a few other races get pissed off, stop by Centauri Prime, and blow the shit out of it - even though he's (for all intents and purposes) the president of the galaxy! Londo tells him what is going to happen to the Centauri... this makes him DIRECTLY responsible for the events of Crusade.
He saw Centauri Prime burning 20 years in the future. Could he be expected to believe an attack in the present was related to what might happen in 2 decades. Also, remember he got Delenn's warning about going to Zha Ha Dum wrong. He believes he'd changed that future by going.
He doesn't make the treatment of the Shadow's allies a top priority. That is a Wall Banger itself.
Sheridan and Delenn's relationship was full of Wall Banger:
When the Minbari civil war was starting, Sheridan implicitly asked Delenn to shirk her responsibilities and stay on the station to keep him happy even though her caste mates were being marched out of Minbari cities to die by the thousands. For some reason, Delenn didn't see this as an utterly monstrous request worthy of giving Sheridan the verbal beat-down of his life or worse.
While definitely innapropriate, hideously so, it's more understandable considering what happened to his last wife. Something she was well aware of. To me the issue was when he suggested sending Lennier instead. Being protective of your fiance is one thing but trying to get her to send her best friend into danger in her place? For your own convenience?
Delenn flirted with Neroon.
Two supposed soul mates fail to communicate, trust each other, or respect each other.
The decidedly non-platonic way Delenn treated Lennier in season 5 despite being married. What. The. Frak.?
Much like with Neroon this is merely an instance of cultural difference. Delenn never acts in a non-platonic fashion according to Minbari standards, the fact that we may view them as such is irrelevant. Though you'd think she might have learned her lesson about refusing to communicate after the incident with Anna.