Ummmm yeah I haven't found much porn that features discreet dungeons. Most sets are pretty much designed for sex so what the ever loving fuck does porn have to do with designing a dungeon room that won't scare Grandma?
Most porn shows dungeons in an unfavorable light or makes them just plain laughable. The comment was rude and really shouldn't have been made, this is actually a great question.

To answer the actual question in a respectful manner: I would have things in my bedroom/living room dungeon that could be different items. For instance I saw a lovely and most non-threatening St. Andrew's Cross that had lots of fittings for taller and larger people. The person who owned it hung flowers from the fittings in hanging baskets and it looked like an indoor trellis. It was painted lovely colors to match her decor and looked amazing. It wasn't until it was divested of plants and turned around that it's real use became apparent!
I have seen large macrame chairs hung from ceiling hooks which could be used for suspension or a sex swing.
Two bondage horses with a length of narrow board can be a bookcase, knick-knack shelf, or behind the couch table.
Large ottomans can hide accoutrements like floggers, paddles, toys, cuffs and tethers. Some even open from the bottom making it less likely your toys will be discovered by nosy parkers.

With just a little imagination you can turn any room into a hidden dungeon!