Well, I am sure that Linds and Christine slept with each other sometime during the 70s. Drugs, hurt feelings from losing their signifisant others and long nights spend at the studio, both fascinated by the talent of the other and the way their voices blended. It didn't mean anything. Everyone shared body fluids back then.
But - there is this interview in the german "Guitar Player" from 87/88. For my dear life I cannot find it since I moved all my stuff, but Lindsey explained that he left FM because Christine asked him to, because she was going to get married. He agreed because he just couldn't stand not being with her anyway. I don't know if the interview really take place (somehow I doubt it) but it WAS written in this paper, I swear.
In that train of thought - I just realized that Christine left the band when Lindsey became a father. That can't be coincidence.
I DO believe that Lindsey admired Christines talent as a songwriter and musician far more than he could ever admire Stevies qualities as a poet. And I doubt that Christine took him as seriously as he would have liked her to.