You know, a part of me feels like the reason Becky asked Moriko to be Makonyan is because Moriko isn't very open/honest about her feelings. The personality difference, that Becky mentioned before, between Moriko and Makonyan may be Makonyan is more honest with herself...?
When Moriko opened up to Becky by rubbing her cheek into Becky's hand, she ended up closing her feelings off when started kissing her. Maybe Becky realized that Moriko will let her guard down while being Makonyan (which is still a little sneaky if you ask me).