I'm a long-term vegan (strict vegan for almost 8 years, vegetarian for 13) and I've only dated people who ate meat. It's never been a problem for me. In every relationship there is a lot of give and take and learning to compromise. Every boyfriend I've ever had (live-in or otherwise) had better respect my choices when it comes to food, just as I respect their right to decide what they'll eat. The only problems I've ever had were based on small mistakes like stirring a vegan dish with the same spoon used to prepare meat, or dealing with their families on holiday dinners.
Cooking can be interesting, but I typically make most of the meal vegan and add in my protein source and let them add in their own. This works even now that I'm cooking healthier. I either make dishes that I know my boyfriend will like (which, frankly, is just about anything

) or I make my healthier version and let him add cheese, sauces, etc. to his own.
That being said, I do know vegans who ony date other vegans, mostly for political reasons or for the "ick" factor of being around meat. I was raised by an animal rights activist so I can understand the mentality, but for me part of making sure this lifestyle was sustainable and liveable was deciding early on that this is a personal choice and something I tend to be a bit low-key about. If I happen to influence someone or educate them on the reasons or methods of being vegan then I'm thrilled, but I find I influence more people by living my own way and letting them see the benefits firsthand.
I think I've had the same experience with eating healthier. Back when I was younger and just starting to lose weight I was very gung-ho and tried to get my family to eat healthier, but I gave that up when I realized that people will end up doing what they want anyway, and that I can only hope to serve as a good example of how it is possible.
I will say, though, that being vegan has helped me a lot in terms of dealing with food temptation. I tend to think about a lot of foods not in terms of food but just in terms of things that are off-limits for me to eat, and instead of limiting me this helps me to focus on alternatives and ways of getting by without them. So if the boyfriend brings home, say, chips, then I have a much easier time telling myself that that's his food, and nothing I need to concern myself with
A good sense of humor goes a long way towards bridging the gap between different food choices.