Anal stimulation of that sort would definitely make a secluded girl like Marle feel things she has never felt before. The emotional cascading on her face does show she's torn on whether what she is feeling is pleasure. There is definitely some pain, which makes it all the more confusing.
Either way, having her vagina exposed to the open air, for anyone to see, and the unrelenting sensation of wetness enveloping her lips by this point would only make her feel more ashamed.
Then again, she is young, shame may not be what she is experiencing, but confusion. The poor girl, she'll get no answers. The orgasms she's bound to receive will shake her to her core. Maybe Crono is watching and she is feeling even more of a responce to his gaze? Either way... these tentacles are doing this on purpose.
A slow anal stimulation to give her body pause, while her pussy simmers and steams for whatever it has in store for her. Look how it ignores her breasts, but ensures her lower region is completely exposed. It's playing with her mind, feelings, as well as her body. For its, and our pleasure.
