IMHO, Mochikura-sensei definitely qualifies as a "Sexy Onee-san", even if for a while she doubts the level of her own sensuality. That makes Douzono-kun class himself as a Man Of Culture for seeing what others perhaps cannot see.
Consider this "match made in Hentai" to be "APPROVED!"
Desperate and a little tired...That's not a great combo when you're also horny and trying to fight back urges, esp when the other person is obviously interested in you.