If we are talking about wedding meals that are catered at a hall or by a restaurant, I think the onus on the person with the dietary restriction to speak up, whether via email, a message on the response card, or through a phone call. One can't account for every dietary preference on the response card, and the pricing/structure of sit-down meals usually means choosing 3 plates for your guests to choose from, but there's always an unsaid vegetarian option for people who don't eat meat. I would not, as a host, choose one of my 3 options to be vegetarian, thereby limiting the vast majority of my guests who do eat meat, when the caterer/restaurant will do a fourth option as a matter of course. Caterers have a lot of experience dealing with dietary restrictions, and it's absolutely no skin off the hosts' back to pass along a dietary restriction like vegetarian, gluten-free, vegan, etc to the caterer who can easily accommodate it. (There's no additional cost, etc. It's all a matter of communication.) I think the story is different when someone is cooking for a dinner party.
For my wedding, I didn't give guests a choice in advance of plates. Rather, we did a "duet" for everyone (a meat and a fish on one plate). Guests who did have dietary restrictions simply just wrote it on the response card, in the form of "vegetarian meal" or "gluten-free" or "alcohol free," and it was no problem just to pass that along to the venue. There are so many different dietary restrictions, simply putting a vegetarian meal on the list wouldn't account for all of them, so again, I think it is the responsibility of the person with the restricted diet to simply communicate that. It isn't rude to do so, because it doesn't create any work for the hosts. (It does for the caterer, but that's what they're paid for.)