I think that there's a simple and obvious answer to this: sit down and have a grown-up conversation about it. It's likely as simple as him not knowing that grassfed is more expensive, and honestly if you're willing to share food with him at one meal why should he think that the leftovers should be off-limits? It's a misunderstanding based on lack of communication- there's no need to be defensive about it. In fact, diplomacy will get you much further if you value your relationship.
Instead of ultimatums, why not give him choices? Explain that you choose to eat quality meat and that it is more expensive to buy. Follow with: "Honey, it's just as easy to cook for two, however being the only one buying the grassfed meat is hard on my pocketbook. I'd like to continue to share meals with you... so would you prefer to pitch in money for grassfed meat or would you prefer to buy and eat conventionally raised meat? Either way I'd be happy to cook it while I'm cooking my dinner." Or something along those lines.
Give him the benefit of the doubt. Anyway, this will be good practice for future financial discussions, especially if you plan to get married/consolidate bank accounts with someone at any point... Best of luck! :)