Highly in favor of this request. In my opinion, it's straightforward; if you have a Like button, there should always be a Dislike button. It's only fair. Just like it's fair to accept something as good, beautiful, or acceptable, it's equally important to acknowledge when something is bad, ugly, or unacceptable. Upvote comes with Downvote.
I'd even go further and suggest having reactions next to scenes, similar to POVR, and make the percentage visible. The current system, mostly linked with the release date, tends to favor older scenes as they accumulate more likes over time.
Downvotes are a way of expressing disagreement, and instead of having a ton of 'I disagree' comments, just put a downvote and move on. This always happens to me on the site, I bump into a scene starring a girl I don't like for some reason, and my intention is to not have her booked again.
So, I don't always want to go and write what's on my mind; instead, I just find someone who said 'this girl is amazing' or 'book her again,' hit the downvote button, and move on with my life. Another example is I don't wanna see any Anal content on here (fuck diversity). All I do is scroll down and find @petermc, who's highly likely already said his famous phrase 'great anal scene,' put a downvote and move on. Sorry, man, I love you, but not Anal stuff.
In a parallel world where SLR produces voyeur scenes, I scroll down to find that @Niteflyte had said 'more voyeur scenes' I click downvote then move on, simple. I don't have to argue, I just don't like it, and putting the (-1) downvote is my way of disagreeing with you.
Now, IMHO anyone who's calling against it is likely due to having some unpopular opinions. Yet still trying to wrap it under other flags. Again, Fuck diversity, there's still too much diversity on this site, which is why it's unlikely for me to get a lifetime subscription anytime soon.