Having run a forum for a while I can tell you the google ads key on certain words in posts.So the Peta ads will keep showing up as long we keep talking fishing.I will refrain from mentioning a certain western state I used to reside in.
The forum I ran was mainly a furbabies one.For women that is with just two males.Everytime sex was mentioned,and that was one of their favorite subjects,banner ads for male protection would show up.They thought it was hilarious.