They'll do it just to screw with us...
I'm starting to like the concept that Ayumi doesn't have a goddess and doesn't remember, she just started spontaneously liking Keima again in spite of her erased memories.
I'm interested in seeing how Keima spins the cross-dressing. Shiori is a pretty strange character herself, Keima knew she would do most of the work inside her own head rather than him having to actively court her.