He's turned my characters down before, but only if they asked him for a little fun instead of waiting for him to ask. I wasn't sure if that was a bug or a feature - apparently it was a bug? It's interesting how willing the companions are to bed each other, but get all picky with the PC.
I thought this was a feature - not a bug. I thought him thinking you're playing hard to get made things more "fun" and seemed true to his character. I had him at Neutral (or perhaps somewhere under Medium Approval) and I had my character act uninterested and he propositioned me and I took him up on it to see his romance scene. I figured if you act too eager it throws off the balance of power for him.
Huh. I got to the party, had Astarion at medium approval, was ready to enjoy the fun rejection line... and he came on to my character just like he always has. I went back, took a different path through the conversation and got the rejection. Still bugged in spots, apparently.
After Astarion says his line about a little fun, I think what you choose in response determines if he'll turn you down or not. I don't remember 100%, but I think the bottom option plays on disinterest which can lead to him propositioning you regardless.