Yeah? Who says so, apart from those with early 21st century sensibilities? This is yet another example of people thinking that today's sensbilities applied to yesteryear. I wonder how many people of the time would have agreed with you that it was "an unjust law".
And whether just or unjust, it was the law of the land and he broke it. If you pardon Turing for breaking the law then you can pardon everybody in history that has ever broken the law. Why don't be pardon the Yorkshire Ripper or the Great Train Robbers?
In that case, why don't we pardon every person who has had sex with an under age child? Nature didn't intend there to be sexual consent laws.
Are you looking for an excuse for your pedophilia?
Quite often a very stupid reason for incarcerating a person. In the City of Winnipeg it is against the law to carry water down the street in open buckets between the months of November and April.
Now that is truly stupid because it would be a block of ice anyway.