Essek remembers one time when he was visiting one of the Assembly members (Ludinus or Trent, either is fine) in the Empire, they brought over a blindfolded human of roughly Essek's maturity level and invited/instructed Essek to use his mouth while they talked.
Caleb has vague, mostly repressed memories from his time in the asylum, of a day that didn't hurt the same way as most of the others, of strain in his jaw and a scent or feeling to humanoid skin slightly new to him.
More than a decade later, Caleb has Essek in his mouth under enthusiastic consent in the research chamber, a poorly timed hair pull causes Caleb to choke, and the recognition comes rushing in.

+ that encounter back then was Essek's first time
+ Essek wasn't in the position to refuse (if it was explicitly stated or just Essek's own headspace making it seem that way is up to the filler)
+ Caleb's reaction is ...worrying. Maybe he first and foremost feels shame about Essek having seen him like that; maybe he's disturbingly quick to accept it, file it away, and continue while Essek is still reeling and expecting Caleb to hate him; maybe it's good old regression again in hopes of not angering Essek—either way, confusing trauma reactions all around
+ putting the comfort in hurt/comfort

- any romanticism of the circumstances under which they met