No matter how long you've been with your spouse or partner, there's still a protocol for sharing food.
A listener, married for 28 years, has finally had enough of his wife taking his food without asking whenever they go out for dinner or grab take-out. She reaches across the table to take food from his plate; sometimes even putting it back if she doesn't like it. And if he says anything about it, she accuses him of being a food hoarder.
The first fundamental of food sharing etiquette is that it's not polite to touch other people's food - even if you kiss this person regularly. And always ask first. Unfortunately, we tend to forget these rules with partners, best friends and family members. So regardless of the relationship, always seek permission before taking food from someone else's plate.
In this situation, though, you could be opening a can of worms. Having been together for 28 years, there might be other unspoken grievances waiting to be launched. So consider having a delicate conversation where you each ask what the other person could do better in the relationship. That will provide the opportunity for you to suggest that she ask first before taking your food.
Remember, even after a long marriage or relationship, you may still need to get to know your partner and respect that they might think a little differently than you. Hear more on the topic by listening to this podcast of Awesome Etiquette.