I now give you this gem of a read, one of my favorites https://m.fanfiction.net/s/11418258/1/Meet-The-Parents
Other works by the same author are amazing and perfectly capture the charecters personalities and interactions.
Dunno, haven't read/seen the series. However, this comment kinda caused me to do some low-quality philosophizing, so if you'll indulge me:
Wouldn't things like narcissism or masturbation also count as selfcest?
I'm watching all seasons in order (after having already started from Kirakira) and am still on Max Heart as far as watching everything in order goes, but I'm already shipping these two and Haruka x Minami. :P