Damn I'm surprised about all these replies, basically calling OP the one in the wrong, he has to change to fit her new lifestyle choice, why is this? He is a meat eater, he describes himself as such, hell it doesn't even matter about his gym goals, why is it he should suddenly have to completely alter his diet to suit his partner? So she doesn't eat meat now and suddenly doesn't like it, she can't expect him to change as well. It's not difficult for her to just put some chicken in the oven, she barely has to touch it at all. I mean what's her problem, some ethical thing about the poor animals? What about the poor animals who's habitat is being annihilated for soy bean plantations, or the significant population dips in certain rodents when its harvest time, whir up those slashing blades boys. Morally you can find issues from either diet, for it is the quantity that we need to produce which harms the environment either way. What if OP got organic free range meat from smaller suppliers, wonder if that would be acceptable...
There's also the fact that this vegan thing is new, she's apparently okay with him eating meat right now, how long do you think that's going to last? I guarantee she will start harassing/shaming OP for eating meat, "you can't keep that stuff in our fridge", type of thing.
That said, OP it really doesn't take long for you to grill a steak/chicken breast etc to add to her vegetable main. My greater concern is for how long you will have that option. So go with it for now IMO, but if she starts controlling what you can actually eat, head for the door.