By default, Lemmy allows downvotes globally. However, when a server disables downvoting, it is similar to using a feature that is usually reserved for enterprises and very small, non-federated communities.
If a user prefer to not see downvotes, they can disable it by his favourite client settings, but the rest of the community should not miss this functionality for the pleasure of few users.
Downvotes on lemmynsfw were being used to bully people who posted pictures of themselves. They weren’t always disabled. But they became a problem and the instance felt that downvotes didn’t belong in a porn instance.
There was no reason to downvotes a porn post. Ever.
If it breaks the rules, report it. If you don’t like it, keep scrolling.
It would make sense if the end result was to prevent downvotes on NSFW posts by any user, but that’s not how it works in practice. My lemmynsfw account can’t downvote anything, but my other accounts can downvote anything (federated).