Sometimes when being honest guys say the wrong thing. Remember that beauty is also about the flaws. There is little in terms of perfection being a standard. If you need to ask him and suggest that sometimes, being honest and being kind are not together all the time. B.
Most guys generally use the ten point scale when rating a girl solely on looks. Saying you're a "B" would mean he thinks you're average or slightly above average. I wouldn't pay it much attention. Post a photo if you'd like more feedback.
He probably wanted to get a rise out of you ... and he did. If he graded you A you'd get bored with him. Now that he's rated you 'B'... you've probably been thinking about it and him all day. It's a dating tactic.
Either that or he thinks your appearance is slightly lacking.
He's basically calling you a 7/10. Not completely perfect but still attractive. Honestly I'd just walk away from him. No guy who is smart enough would openly tell a girl that she's less than perfect even if that's not his opinion. It just causes drama, although it is more realistic.