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Yaoi has more emphasis on the sexual relationship between two males. So stories focusing more on sexual desire and the sex it self between two guys is classified as Yaoi. (Yaoi literary means "No Climax, No Point, No Meaning)
Shounen Ai was another form of Shoujo created in the 1970's as a form of friendship/ growing relationship between two males. So usually anything that has more emphasis on the emotional aspect is considered as Shounen Ai. Basically it was a type of literature that could focus more on the emotional aspect than the sexual desire.
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Shounen = young boy
Ai = love = Boys Love (BL)
Shounen ai will have kissing and implied sex at the very most.
Yaoi, as a genre, is geared to those 18+ and is generally considered porn.
Yaoi can contain explicit sex scenes, censored sex scenes, nudity, adult themes, adult language, violence and fetish content.
Wow! I really enjoyed this yaoi, like its the first yaoi I've come across where the boys aren't thinking of having sex everywhere. This is really cute ^^