A domain value is more than just its search volume.
The biggest problem with so many adult domains is when you take the keyword phrase of the domain and do a search in google.com.au (unfiltered) you get international result. Turns out Google reckons most Aussies aren't necessarily looking for local porn.
My tip - ask yourself is this domain able to have a small business built around it? Do I get Australian search results when I do a search in google.com.au? Are there lots of Australian advertisers in this niche?
Personally I'd sooner buy AdultToys.com.au before GroupSex.com.au despite "group sex" have over three the number of phrase matches that "adult toys" has because Adult Toys you have a sellable item and most Australians will look to buy from Australian retailers.
The value of a domain always comes back to the potential value the person buying can derive from the domain. So if you can't see yourself developing the domain into something that would generate value then you are probably going to struggle to convince someone else who's intellegent that they could.