Maybe the word "yaoi" didn't exist until the seventies, but man/man romance is pretty ancient in Japan, I'm sure of it.
It actually used to be considered the purest form of love, much "worthier" than any relationship that involved women. But when Japan started interacting with Western countries, which considered homosexuality a terrible sin (or even downright a crime... In some countries you could go to prison for being homosexual), Japan white-washed itself to fit in. IIRC, for awhile it even outlawed homosexuality.
Of course, it was about as effective as outlawing alcohol...