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Either it happened in the moment and Adam didn't deliberate on it very long to realise it's a bad idea. We know nothing about the fruit asides what it did to them over an unknown course of time... robbing them of their veil (whatever that is exactly) making them aware of their 'nakedness'. The fruit could have been like heroin or a powerful drug something like what we know, the first few eats might have sent them into ecstacy, maybe imbued them with certain gifts that they believed more and more the devil's lie. But they became addicted and starting needing the fruit. After a while they became like gollums.
That or Adam did reject Eve's invitation at first but after seeing that God had (seemingly) not found out, and Eve was (perhaps) getting wiser or more beautiful or something began to entertain thoughts that maybe God was deceiveing them.
There's a lot about the story we don't know. Like how long were they eating the fruit before God revealved that He knew (of course He was watching the whole time). I know if I was in God's poisition, after they'd eaten the fruit, what I'd want to know next is how many times are they going to eat it, and will they confess to me of their own accord, will they lie if I enquire about it, to what extreme will they go to conceal their transgression if I go wandering near the tree.