I think the most effective counterargument to this type of "everyone should believe and do what they want" argument is that it would be true if one's actions didn't affect other sentient beings, but that's not the case when it comes to food that contains animal ingredients. Personally, I'm not one to believe that non-human animals should be given as many rights as humans are given (I'm sure some on here would disagree), but I do believe that they should be given some rights, including the right not to be forcibly bred, raised in captivity, and in many cases effectively tortured, and slaughtered, just to produce food that's unnecessary in the food supply. So, that's how I'd approach an argument with someone who was using the meme that "hey, diet is all just a matter of personal choice." Namely, I'd point out there is another being involved, which may not be as intelligent as humans are, but is still sentient and capable of suffering, so that the "personal choice' concept is inapplicable.
On the other hand, I'd make the point that this is what I believe regarding the use of animals, but I wouldn't keep talking about it, nor would I mention it at dinner, because doing so doesn't change people's minds. That's how I deal with it with people in my own life.