I run an inspiration travel website, unrelated to porn. Yet, Google Webmaster Tools reports that many different pages on my site are showing the top 10 results for porn-related queries. There is no content on those pages being related to porn.
When I put the query into Google search myself, I never see one of my pages showing in the search results. What would be driving this? Is this a bug or something malicious?
It looks like your site has been hacked. This typical hack is injected in your htaccess file and is only visible to search engine bots. So, they see different content whereas the visitors see something else.
The searches may not be normal web searches. Webmaster tools also reports when people do image searches. Also check Google image search and see if you are showing up there.
Otherwise, Krinal's suggestion that your site is somehow hacked is probably correct.
Check your website backlinks profile.There must have been anchored links point to your website with the that porn keywords. (Remove those links if you can or otherwise use disavow tool)
Check your website alt attributes for images and see if there is any hidden text from where Google might have been picking up the relevancy. (remove content that contains that keyword)
