Whilst I appreciate the sentiment behind this, it is really flawed for several reasons:

1. Not all rapists have testicles. 
2. Rape is not normally motivated by a person having a high/uncontrollable sex drive. Castration will not change the need for power and control that usually does motivate rapists. 
3. Castrating someone is probably going to make them rather angry, and this is likely to exacerbate the feelings that make them want to rape in the first place. What’s more, a lot of male rapists are motivated by feelings of emasculation. So, yeah, castrating someone like that is actually rather risky.
4. Castration doesn’t address what predominantly underpins sexual violence in society: proprietory attitudes towards women, rape culture, misogyny, etc.
5. It’s naive to think that sexual behaviour is entirely governed by hormones.  
6. It is still possible to offend when chemically castrated. 

Whilst castration can be effective for a minority of offenders (some actually request they be castrated, and I have no issue with this) the vast majority will be unaffected or will possibly become more motivated to offend. Surely that’s something nobody wants.

Yes, I am aware that there are a handful of studies which show these drugs to be effective at reducing recidivism. However, researching this is inherently flawed and problematic. Firstly, it’s not possible to have a control group because the effects of the drug are so obvious. What’s more, offenders who know they are being monitored for research are more like to be on their “best behaviour”. Not to mention that the kind of offender who volunteers for treatment is also likely to be more responsive to that treatment - whatever form it takes - because they have the motivation to change. 

I’ll end with a quote from Frances Crook of the Howard League For Penal Reform: 

Sex offending is often not about sex at all, but about violence and domination. The [chemical castration] drugs used will not affect those attitudes. Some men may inflict other types of deviant behaviour on victims if they are unable to perform sexually due to the drugs. The physical impact of the drugs can be very powerful, with the effect being similar to a sex change. ”Feminising” sex offenders may make it more difficult for these individuals to reintegrate into society, which in turn makes their reoffending more likely.

I also think it’s also rather problematic to be using testosterone blockers as a form of punishment, particularly when considered in light of trans people who have to constantly fight for access similar forms of medication. It’s a massive problem that cis people are gatekeepers to medicine that trans women often want and can’t get. And it’s kinda fucked up to be using that same medicine as a way of punishing the most deviant members of society (made worse when you consider the way trans people are routinely cast as deviant or perverted themselves). I dunno, I can’t really articulate this point well, but it makes me really uncomfortable.