With the action on the flop I think you have to assume one of the two is on the flush draw. Also on that board someone with a flush draw almost has to have a str8 draw or at least one over to the board or a pair plus a draw, so a bet makes a lot of sense.

If he had A7 or A9, you're beat on the turn, so I guess you're putting him on K9 or K7? Not likely hands to have called a raise with pre flop.
You took a stab on the flop, I think once you get raised you're done.... you're getting good pot odds but you have no idea what card you're rooting for.