I have many friends who have restricted diets either because they want to or because they have to. This doesn't stop me being friends with them or inviting them round for dinner as long as they are reasonable (and they are). I think with a partner it's a little bit different.
I couldn't see an ethical vegan wanting to date me to be frank, because I'd think the first time I ordered foie gras might be an issue
but I'd be willing to give it a go.
Food is important to me. If someone is open to new things (in as much as they are able), willing to give and take (DP doesn't do shellfish but went to a Michelin starred seafood restaurant with me for my birthday. They worked around his shellfish issue and we had a lovely meal. I'm sure he would've preferred a steak, but I reciprocate by cooking / eating food that isn't my first choice).
Incidentally we both have IBD, so sometimes eating out isn't an option. Hell, sometimes salad isn't an option. Sometimes what works for me (low carb) isn't what works for him (sugary drinks and starchy carbs) if we both flare. Shit happens it really really does, IBD isn't pretty and we deal with it, but I do think there's a difference between ethical restrictions, health kicks, health issues (allergies, IBD, whatever) and just being a fussy bugger.
Clearly, 10 years in if DP wants to become an ethical vegan that's fine. As long as he doesn't expect the same of me...