Their behavior is weird, they act like kids half their age who only just learned about naughty things and try to imitate them.
The guy is childishly pushy and needy, while also not being pushy enough to go for the main course.
The girls are completely unguarded, unable to say no, unfazed by barriers being broken, have a warped sense of friendship, and always go along as long as he begs for it bit by bit.
And they don't seem to have normal libido. They start things randomly and stop randomly, driven by curiosity and pleasure rather than raw desire.