Sean is a pretty big carnivore, the kind to have not one but two burgers a meal. So I was pretty shocked when he said that the time he spent with us in December, he wants to eat exactly the same diet as I do! (I am not vegan: I will have eggs and sometimes honey. I'm allergic to dairy but I "treat" myself to some very occasionally).
I never push my diet because I feel it's a very personal thing. I know that no outsider could have "convinced" me to become vegetarian, it's a decision I had to make on my own. So I don't push other people as I feel it's really not my place. But I've found most people want to at least try, and a lot of them reduce their consumption. Some of them have actually stopped eating animals completely! Sometimes it feels like just seeing it's possible to eat a healthy vegetarian diet is all they need.
I wouldn't make it a condition for my partners. But I never "select" my partners: I fall in love with someone and take them as they are. It means a lot to me though when they want to try eating the same food I eat.
My husband isn't vegetarian either, but he avoids cooking meat at home if I'm around, because the smell can really linger, and while it used to be a pleasant smell for me when I first became vegetarian, now it's rather unpleasant. I don't mind when I'm in a restaurant or at someone else's place, though, if everyone else is eating meat (although I always feel bad if people cook a special meal for me!).
However, there is something nice about being able to eat from your lover's plate, for sure. Or being able to cook for them.
And I like telling people that it's not "all or nothing". If you eat some meat, you're not vegetarian, true, but you don't have to eat meat at every meal or none. You can have one vegetarian meal every so often, or one non-vegetarian meal every so often, or anything in between. It's really about what works for you.
While I guess I can understand wanting to be with someone who shares your values if they're essential to you, I also like being able to share new things, make my partners discover things. There is so much in vegetarian cuisine, I love cooking for people who aren't vegetarian and see their reactions.
However, I think it would be a bad idea to enter a relationship with the mindset of "I'm in a crusade to convert them to X". If it happens, it will be because they want it to, not because you made them, so it's good to keep in mind that there is a chance they won't change at all. If being a vegan is a very important condition to you, don't count on "turning" them vegan. Then, yes, it would indeed be safer to find someone who is already vegan to begin with.
Anyways, that's all very general, it didn't seem to me in your post that you're in any "crusade" about E