Given he's a chef, I imagine he took it as if you were saying you don't support/believe in his profession
What utter bollocks. There's endless non meat meals to cook, chefs who never cook meat and restaurants that don't serve it.
OP, my attitude to meat sounds a lot like yours. I'm not vegetarian but don't particularly like big lumps of meat and some of my favourite meals happen to be vegetarian so that's what I mostly cook or choose when we eat out. Plus I'm becoming more aware of the environmental aspect of meat production, so making a more concious choice to avoid beef in particular where possible.
Or I cook things that only have a relatively small amount of meat for the meal, for example a pilaf that has one chicken thigh per portion, along with chick peas, veg, rice etc. I'm also a soup fan and one of my favourites is a minestrone type, where I use one of those small packs of panacetta cubes in a big pot along with veg, beans, soup pasta etc.
If you occasionally eat meat because you sometimes cook it, or have it as a takeaway you're not vegetarian, so if he still decides to divorce you then it's not about the eating habits then is it?
If I were you, I'd cook what I wanted, not what he wanted. Just say - I'm making X, do you want it? After all, he's always free to cook himself a steak or whatever. Plus being a chef, he probably gets lots of opportunities to eat at work?
Oh, and ignore all the shit about protein and nutrition, Mumsnet is ridiculous about this. There's probably exactly zero people in the UK/developed countries, who can afford to eat sufficient food to not be hungry, who are protein deficient. We don't need anywhere near the amount that people think we do.