Now i finally start to understand what happened in chap 54.
Apparently those fujoshi knights made a yaoi report about the dragon slaying incident. So Tuka turned into a boy and when Yao asked Itami why he refused her request even though she offered her body in return. Itami said he came here to save Tuka (who is a boy elf in this report) not for Yao's body.
That explained why Tuka appeared as a male in chap 54.