it could be because you browsed the forum with a subject about bitcoin, or a user having bitcoin links?
or it could be a related ads.
Once, I had NSFW ads on gbatemp, it really irritated me because I didn't visit such websites and was a problem for me to have these pictures.
Costello said gbatemp wasn't responsible, and the ad is related to google search (not the website you visited, but your
search string !)
I did search a word which could be assimilated to that ads, but it wasn't what I wanted.
maybe you search "bit torrent" and it links it to "bit" something ?
Connect to your google account, there is a place where you can list all your search history
(that's very nice to have access to that history, sometime it saves times finding something again)
locate a search string you did which could be related to "bit" or "coin" or "mine", and delete it from your search history.
now, reloads gbatemp and the ads will be gone/different. if not, you didn't delete the correct one.
edit : That worked because the ads was from google adsense, of course. if the ads you see is not from google, it won't have any effect.