Some folks in the search space feel that adult sites, pornographic sites, are automatically considered to be spammy by Google. Truth is, while there is a ton of spam in those industries, not all adult sites are spammy.
Google's John Mueller said so in a Stack Exchange thread. He wrote:
Also, "adult" doesn't necessarily mean that it's spammy.
That being said, many adult sites are spammy because the adult industry is very competitive. But Google has been targeting that spam with their Spam algorithm.
Forum discussion at Stack Exchange.