"Women A committed the same crime as Man B. Woman A got a fine / community service. Man B got a prison sentence. Not because of any difference in the gravity of the crime, but purely because of their gender".
I agree that this is a bad idea - both unfair and impossible to implement - which is why it'd be interesting to see if this is actually Swinson's plan and whether she has some data to support it, or whether she's just pulling stuff out of her arse.
The data you found on male vs female sentencing patterns is very interesting BrainFart and I'll read it in more detail later when I have time, but the information I was actually looking for was the male equivalent of this site:
www.womeninprison.org.uk/research/key-facts.php
Which says things like:
"the more previous custodial sentences a woman has had, the higher the reoffending rate"
If this is true - and your goal is social rehabilitation rather than just punishment - then it clearly makes sense to reduce the number of sentences a woman has had. Is it also true for men? Common sense says probably yes, but if for example there's evidence that men who serve custodial sentences are less likely to reoffend then it doesn't make sense to extend the "no prison" policy to men and women - in fact the opposite would be more effective.
You're looking at a prison sentence from a purely punitive point of view, using a crime + gravity of crime = length of prison sentence equation. In which case no, sex shouldn't make a difference - input same crime + seriousness, output same punishment. But if Swinson is using an equation that centres rehabilitation rather than punishment, then sex might very well matter. If your equation looks like crime + most effective method of rehabilitation = lowest rate of reoffending, then you have to allow for different demographics to respond better to different methods. For women it might be that drug possession + community service = unlikely to reoffend whilst for men drug possession + community service = very likely to reoffend - in this case it's not a good idea to apply the same methods to both sexes.
It's very well and good to say "I don't see sex/ race etc etc" but we live in a world built on structural inequality and we need to see those things because policy is most effective when it treats different groups according to their different needs. If you treat every one the same you may get an equal society, but you won't get a fair one.
In conclusion, we need more detail on what Swinson is aiming to achieve and why she thinks this is the best way to achieve it.