It depends on the person you're dating. I was vegan for over 10 years and never forced my friends to follow my eating patterns etc. Two days ago, however, I responded to an invitation from a woman who has been trying to get me to attend her events for months. Every time she insists that she will cook an intricate vegan meal and then no one wants to come (mainly because they're not vegan) so the event gets cancelled.
The one event that didn't get cancelled is the one I attended. We went bowling but were not allowed to eat at the place (though it was a very posh place with great drinks, etc.); we were only allowed water, which was a drag. We were then directed to a vegan place for dinner and FORCED to sit in sections that she pre-labeled "Vegetarian" "Vegan" and "Other". She held the menus hostage and took 20 minutes to lecture us on how to order, as though none of us had been there before (or eaten before), then ordered us to leave a certain level of tip. I hated that experience and swore never to attend any of her events again.
So you see it depends on how regimented the person is and on whether you will be forced to follow whatever it is she has in her head. I'm not big on the "my way or the highway" type of people.
I'm not dating a vegan but he's cutting atm, which means he only eats a small amount of oats + steamed veggies. It's working so far. I'm cool with it as long as he doesn't push his eating habits on me or make me feel bad about the food that I continue to eat.
Well i don't know about vegans but i'm a vegetarian and i don't care what you eat. In fact i could care less. I wish more vegans just chilled the hell out. Same with some vegetarians. They make it seem like a religion and veggies their god. Lol Take care. :)