Started with K/S. As soon as we bought a computer I searched for Star Trek fanfiction. I knew it existed, knew slash existed, but had never been able to get my hands on any. Didn't take me long to find dozens of sites. Didn't take me much longer to find the slash stories. :-) And while it's no longer my only fandom, it's still my main one. Kirk and Spock are the two I grew up with. I was hooked from that first episode 41 years ago, even if it was only seeing them as best friends. So I know them in a way I doubt I could ever know any of my other pairings.