
I just imagined a possibility that will probably never happen but will haunt me..What if Rei switched to Itsuki while they were doing you-know-what...(⊙…⊙ )

It can be in quite a lot of cases. Triggers can set off certain memories or thoughts that can sometimes make a personality want to come forward (take control of the physical body). Sometimes triggers can be negative and force personalities to come foward. It's different between people.
I am not a profesional I just am friends with someone who has DID.

Spoilers
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There will be scene like that in 2nd volume. While seme was rimmimg uke he switched to Itsuki... And Itsuki was grossed out why he is licking some guy's asshole. Started spitting, in general was rather disgusted.
Tarou-san is so cute and kind (▰˘◡˘▰)