[UPDATE: I have created an NSFW version of this post. Here is a link (and I hope your app/browser can translate from my home instance to yours)

I got an official answer on this one.

You cannot currently post an NSFW reply to this post because this post is not currently tagged “NSFW”.

1. You can certainly post your NSFW response as its own separate post on this community, as long as you hit that checkbox saying “NSFW.”
2. Alternatively, I can also edit the post, checking the NSFW checkbox specifically for this post
3. I can try to post a version tagged NSFW