Would it be fairly safe to say that if we have a sin nature, then it's not a physical reality, given what we know of genetics? This also means that Paul's teachings, for example, concerning 'the flesh', don't necessarily correspond with the flesh, even though we know that the object of sin is usually rooted in physicality (but in a physical world, wouldn't this be expected?)
In all seriousness, though: Eve was deceived, not Adam. Eve misquoted God's command in Genesis 3.3, but Adam -- he disobeyed, fully aware that he was disobeying. He, unlike Eve, hadn't been deceived. If Genesis 3.6 is understood to mean that Adam was with Eve while the serpent was speaking with her, then it's all the worse for him, as now he's failed to correct Eve, or intervene in the exchange in defense of her. If we're 'in Adam', it's because he sinned in full knowledge of what he was doing, while Eve did not.