i watched the zone cartoon with starfire and raven in either elementary or middle school. i hated the fact that i enjoyed looking at a half-naked alien that spoke broken English and a goth grey chick getting turned into a sperm bank so much, i made excuses to go onto the page itself and look at it. like "im going to make a newgrounds account so i can give this a low-rating" or "i want to make sure i see the URL so i dont accidentally click this links while out on other websites"
looking back i think i got some trauma from looking at that from a young age.
Growing up I watched Pokemon and Naruto. The female characters made me rock hard, I felt the urge to insert dick, see them naked and getting fucked like animals. So I googled for porn of them, and thus I learned porn with anime characters is called hentai. BAM, jackpot.
Thinking back, that was a long time ago... there are a few things that pop into my mind but I am unsure which occurred first:
Newgrounds - that one series of Sex Kitten games
Imesh - just downloading random pictures and clips pre-limewire and coming up with every variation for the word boobs that I could think of to get different results
Related to the above was a weird Love Hina Hentai which might be the first actual hentai I enjoyed?
Honestly, i find anime artstyle to be pretty damn erotic, even when i was young. As soon as i saw my first anime with some cute moe artstyle (first anime was baka to test i think?) i got hooked pretty fast. Things kinda went south pretty fast after that.
Highschool DxD probably... actually might have been SAO way back in the day, I don't quite remember. I do remember the first time I saw an anime titty though, it was Air Gear, on some sus dubbed anime sight I can't remember, and I turned it off so fast thinking it was actual porn lol.