^The trope of giving weak denial to shameful acts is a Japanese matter in general. It is seen as proper for a woman to not show that she is into lewd acts. Therefore it is just expected in porn that the woman gives lipservice about being ashamed until "pleasure overtakes her and makes her honest". It is less rapey and just deeply sexist in origin. This has become so ingrained of course that it happens in Yuri just as much.
This triangle has been portrayed this way for a while now. Mostly because Fauna has admitted to having interest in both of them and not feeling jealous about it.
There are a lot of interesting details in this piece, like the girl in the back having freckles and wearing a Chinese dress, when everyone else is in western clothes, and the girl in the front heavily blushing in the mirror.