My fella and I have been together for almost three and a half years, and he's seen me go from restrictive-type bulimia to vegetarian to Paleo. I'm very glad that he didn't decide to ditch me for not conforming to his SAD diet. ;)
Yes, I'm concerned about his health. I think, if his diet was of great issue to me, that would be the motivating factor to not pursue the relationship. I would hesitate to date a chain-smoking, non-seatbelt-wearing, corpulent alcoholic, just because I'd be worried about longevity (and yeah, I guess the attraction thing plays a part, too >.>), and I regard SAD on about the same level. However, I love him, not the food he eats.
While his diet isn't a definitely not a deal-breaker, I'm still taking measures to improve his health, though, just because it is important to me. I'm currently trying to ease him off gluten. I'm not nagging him or making a big deal about it; I simply haven't prepared him any meals with gluten in it for over a month now (and not necessarily on purpose or deliberately deceptively, either; I merely do not regard gluten as food, and therefore do not cook with it). I finally pointed it out to him about a week ago, and he seemed pleasantly surprised and said he loves the food I make for him. I was also thrilled when I looked back and realized he had eaten entirely Paleo on Sunday without even trying. It just happened spontaneously, which is how it should be. :)