Is it okay to criticize your significant other's appearance?

(Is it okay, or worth a damn to criticize anyone's appearance? Probably not, but there's a reason I ask.)

My feeling used to be that it was okay to criticize aspects of their appearance that are within their control, as long as it was done constructively, but ONLY if they asked for your opinion.  And I also believe in Dave Barry's maxim that a woman should never, ever ask a man how she looks, because we all know there is no correct answer.  I may be one of few women who actually lives by that rule, too.  I'll ask a girl friend, if I'm really unsure, but never a man, least of all my significant other.  (And really, as a costume designer, I feel I ought to at least have confidence in my own taste in clothing. Besides, those of us who know how to dress, know that if you hear remarks on your appearance without asking for them, you've made some kind of impression.)

For example: A boyfriend of mine took me to dinner at a fancy restaurant one night.  He was wearing an olive green shirt with a bright red Hawaiian print tie.  I didn't say a word, even though the combination was atrocious, partly because I knew they were the only nice shirt and tie he owned and he was making an effort, plus we hadn't been dating that long and I didn't want to scare him off.  Later in our relationship he actually asked for my help constructing a suitable outfit for a high profile job interview and at that time I informed him that the red tie was a no-go with the green shirt. To be honest though, I think even if he hadn't asked, I wouldn't have let him go to a job interview dressed in that combination.  Because I loved him.

Later on, with the same boyfriend, it came to my attention that he had put on a significant amount of weight over a fairly short period of time. He was a heavy guy and had been overweight before. This was never a factor in determining my attraction toward him, or I  wouldn't have dated him in the first place.  However, I was beginning to worry about his health. Really.  I felt eventually that I needed to tell him.  I did it with as much consideration as I could, and I think I managed also to do it without sounding condescending, although I may be wrong.  Regardless, his feelings were deeply hurt, and he told me so.  I don't think that necessarily means my worries weren't valid, but the experience taught me that it was a much more delicate subject than I'd realized.

I have since then come to believe that people who are overweight don't need to be told about your concerns for their health.  They've heard it. Some of them are fine with their body as it is, and simply don't want to be harassed about their priorities in life; others are not happy about it but lack either the determination or will to take steps, and still others are in fact perfectly healthy.  I no longer believe it is my job or anyone else's to put pressure on people to slim down.  Life is short, maybe some people would rather eat cake, and that's their business.  Nevertheless, in a long-term intimate relationship, I think it is important to be able to communicate your concerns about the overall well-being of your significant other, although I guess I don't know how to do that yet, if being constructive isn't enough...

So, the reason I bring up this subject is this:

Yesterday my current boyfriend (different guy from those two anecdotes) and I went to the dentist together.  I was surprised to find that in the six years since I'd last been there I had no cavities.
They took x-rays and saw that my wisdom teeth seemed to be pressing on the rest of my teeth, which didn't surprise me, because I'd had one removed once for that very reason.  However, I wasn't in pain and although I do have a slight overbite  (very slight) and always have, the dentist said really it wasn't necessary to do anything right now. She did suggest that I might consult an orthodontist about the wisdom teeth issue, because braces might help space my teeth more evenly. I pointed out that if my teeth started hurting I would just get those wisdom teeth removed and that should take care of everything.  She agreed.

Later that night, my boyfriend and I discussed the results of our visit. We got on to the subject of my teeth and the dentist's suggestion about braces.  I stated rather confidently that I thought they were unnecessary. He said, "I don't think so."  I was stunned.  I told him, "But there's nothing wrong with my teeth, they work fine, they aren't causing me any problems, so why should I get braces?"
His opinion in a nutshell was that my two front teeth are slightly crooked. I was hurt, and utterly startled. I would never claim that my teeth are perfect, but I don't think they're noticeably crooked, and certainly not distractingly so.

No one- not a dentist, orthodontist, or anyone at all- had ever accused me of having even slightly crooked teeth. Even the people back in school who hated me and made fun of me for everything they could think of (I was spectacularly unpopular as a kid) never mentioned this flaw. They made fun of my name, my mannerisms, my butt, my legs, my bellybutton, my religion, my family, my ideas, even my posture, but no one ever made fun of my allegedly crooked teeth.

 My boyfriend's feeling on this was that I should be thanking him for his honesty, because only a true friend would be so straightforward, but I really didn't feel the same way. If I had asked for his opinion of my teeth and he had told me he thought the two front ones were a tad bit crooked, maybe then I would thank him for his honesty.  Or, even though the criticism was unsolicited, if I were looking into a career in acting or modeling.  But I'm not.

I asked him exactly what favor did he think he was doing for me by telling me this?  His answer was that some people might discriminate against me because most people associate crooked teeth with stupidity.
This only upset me more; he wasn't concerned that my teeth might cause me physical pain or even physical difficulty! No, he was concerned that some insensitive person might be critical of his poor crooked-toothed girlfriend. 

Even though he thinks I am beautiful, as he was quick to add.  Eventually I asked him directly if my teeth bothered him aesthetically and he said no.  He also proceeded to tell me that all his friends thought I was totally hot and had said so at a party we went to recently.
 Why doesn't that make me feel better?

The question I have now is, does my dear boyfriend want me to return the 'favor'?