At first glance it seems quite reasonable - but the law was changed in 1967!
It's not possible to hop into a time machine and 'right the wrongs' of the past, so I wonder about the motive of people who propose these kinds of laws. It's not as if the government has nothing else on it's plate.
The law changed in England and Wales in 1967, in Scotland in 1980, in Northern Ireland in 1982.
It's a Private Members Bill so is not distracting the govt. Private Members bills have no chance of getting passed if they were to seek to change govt policy, say to abolish the bedroom tax, as it will be voted down. It has a much better chance if it is something that can gain cross party support.
As to John's motives, growing up in Scotland in the 60s and 70s, as a gay man, I think they are fairly obvious. Will it right all the wrongs of the past, no, but it will mean that we recognise that people should not have this as a criminal record. It's a small step along the road in treating homosexuality as normal part of life, but it is a step.