Well, It started on other forums for me, the Tf2 SPUF and TerrariaOnline.
First, I'll explain the Tf2 side.
When I first bought the game, I hit the quick join button, sending me to a random server. The server info page contained the usual things; No spamming, no griefing, that sort of thing. But at the bottom was "No furry sprays". At this point, I didn't even know what Furries were. Eventually, I was consumed by curiosity and asked in chat "What are Furries?" I got a few "Do you live in a box?" type comments, but then someone told me it was just animals having sex. I kind of went "ick" and moved on.
Then I found the Tf2 SPUF.
Oh, the hatred of Furries there is overwhelming (You wouldn't blame them if you saw the Tf2 Furry community, but that's another story.). Anyway, I was recommended to a Furry server. I entered, and my eyes recoiled. Furry porn (and some non-furry) adorned every square inch of the map. Another ick moment. I left the server.
Of course, that's just how I got to know that Furries existed. The TO part is next.
Ah, TerrariaOnline. A nice small-ish forum with a core of friendly regulars. This section is a lot more positive. Several people on there have furry avatars, and myself being a regular, it was only a matter of time until I found one. I saw someone who's signature said "Thanks to [Name_I_Forgot] for making my fursona!" Being the curious person I am, I googled Fursona and found out what it was. I thought "That's cool! I want a fursona!" So I drew myself a fursona of a Bearded Dragon, and went to Google Images to see what others' were like. I expected... (continued in next section)
The last section was getting kind of long, so here you go.
...to be assaulted by rows and rows of porn images, but I just saw images of people's fursonas. I was interested, and looked for some forums. I found them, and after lurking around here a few days as a guest, I finally decided to join and made myself an account here.
Wow that's a long post. :0