I feel that sexual attraction is much more subjective than love, and that while you can't fight the feeling, or lack thereof, in either case, the nature of standards usually attached to sexual attraction is what makes people call this shallowness. However, I'd be curious to see how willing all of the people who have called you shallow over this would be to sleep with somebody who is the opposite of what they typically find attractive (as in your blonde/brunette example, but we can extend it to a lot of things).
You're not questioning whether the worth of a person varies with weight loss or gain, or their worthiness to be loved --that would be shallow; you're questioning sexual attraction after weight loss/gain, and that attraction is determined by instincts we keep as animals even more than by a conscious choice we might make.
In your particular case, I feel that the contradiction in lifestyles is a bigger problem than the weight gain, in that if the 2 of you were to carry on dating, I only see 2 options: one of you would get frustrated over what's for dinner, or you would need to make separate food every time. The former seems especially hard to sustain, and I don't see how that would not generate conflict over time, and possibly the habits of one of you wearing off on the other one (sure, one can hope that your healthier habits contaminate him rather than the opposite, but there's always a risk). To me, the nutrition and lifestyle discrepancy is no different than a new couple involving a person who loves total silence, and a person who slams every door, plays loud music & TV all the time, and so on. I'm not saying it can't work, but you really have to love someone to stand things that you otherwise loathe just because they do it. And I'm saying this as someone who's been in the same relationship for over a decade, living together for 2, and... have to keep everything separated in the fridge & maintain a kitchen schedule, because we eat completely opposite things (I need my vegetables, legumes and spices, with meat a maximum of twice a week and no eggs/very little dairy due to some health issues; he eats meat and eggs at least once a day, only salad or cucumber for vegetables, and the only fruit he likes is a fruit that triggers IBS flare-ups in me. Trust me, sharing a kitchen in those conditions is a nightmare, and I wouldn't do it if I weren't in love).
I'm concerned, however, with the reasons that he no longer hikes or eats healthy foods: had he been lying about it before, or was it true, and for some reason, he's given up on everything (the guy might me clinically depressed for all I know)?