I guess more power to Yuna for doing whatever she likes, but for me, this video doesn't do it. Thanks to the uploader but let's discuss what this video could have been.
Imagine a version of this video, where Yuna actively doesn't want to cheat on her boyfriend, and where this Osuke guy remains active and assertive throughout the scene.
Instead of following them to the onsen, only to then passively wait for her to make the first move, he should have made lewd comments, and openly stared at her fantastic body, making her blush and become irritated in equal measure. The script is super important, and it's a shame Dob doesn't pay it more attention, especially given how cheap it is - words don't need rendering.
Had he inserted himself close to her, and later giving her no option but to leave the water naked only to follow her to corner her, daring her to call for her boyfriend, and when she hesitates, simply fucking her... now that would have been hot.
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For "cheating" and "cuckold" to be exciting there needs to be a forbidden element to it. I guess you could be exited for this video if vanilla sex is your thing, or if you're cheering for Yuna to have a liberated sex life where she's in control, but fetish-wise?
Weak.