Junjou Romantica by Nakamura, Shungiku 
Read article here http://reebekillustrations.com/2018/12/18/first-bl-manga-you-ever-read/
I remembered the first BL read was in my teens. I would regularly go to my local Waterstones bookshop after school to read books. Waterstone was introducing manga books in their stores.
TOKYOPOP was the main manga publisher at the time. The first manga I brought was DearS. (I still read that manga)
There were other manga books too. I picked up Junjo Romantica.
When I first read Junjo Romantica for the very first time, I was shocked by the graphics detail of the bedroom scene. I was taken aback by the rawness.The drawings were different, unique to other manga styles. I never knew this kind of manga existed in a book to buy. It was very entertaining to read. I blushed and squealed reading this manga.
Since then I been obsessed with reading BL mangas.
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Please read my featured article: Boy Love (Yaoi) Manga Recommendation to Buy at CDJapan featured on CD Japan website: https://www.cdjapan.co.jp/feature/Boy_Love_Yaoi_Manga_Recommendation_to_Buy_at_CDJapan