Not that my "vote" counts for anything, but having been involved in an extremely long and contentious thread once in another forum (completely unrelated to the topic of this one, much more "mainstream" and "non-adult" in nature), in which an entire extremely long thread was deleted, after months of posting by many members, possibly a year or longer, some, including myself, having spent hours writing very long and carefully composed posts), when the OP apparently accidentally deleted his own first post -- or, that is, either accidentally when attempting to edit it or not realizing that it would delete the entire thread with it, or so he told me in a PM -- an incident which prompted the admin, who felt extremely badly about the thread's deletion, to disable the members' ability to delete their own original posts, I've formed a rather strongly adverse attitude toward what in my view are often relatively "frivolous" deletion of threads, in which any potential problems involved can almost always be addressed in a less extreme or destructive fashion.
Again, not that my opinion matters, but I feel strongly enough about it to state it anyway. Although I have little choice but to accept a moderator's decision in such matters, since people often spend much time and thought in composing their posts, many often spending much more time and effort than the OP him/herself, IMO complete deletion of a thread (as opposed to possibly just locking it) should be an extremely rare occurrence no matter how strongly the OP or other posters may request it.
In fact, the only two situations I can even offhand think of in which I might consider it fully "justified" are either when something is involved which realistically might likely have serious legal consequences, or possibly in cases were someone has either intentionally or inadvertently revealed very personal information about him/herself or others which might seriously compromise someone's safety or privacy -- and even in this latter case, editing of posts might often be a far preferable alternative to deletion of an entire thread. Oh, and of course when an OP is obviously nothing but blatant spam. But, FWIW, anything beyond that is over-moderation, IMHO.