This is a question that came up today in an online forum, but it's one that I struggled with years ago. My grandfather had taken me out to eat many times in my life, never dictating what I could order and never judging what I did either. When I became an adult and had the money to reciprocate, I wanted to take him out as well. But I was faced with the question of whether or not I would want to pick up the tab, because he would most likely order meat.
I thought that question over a lot and I decided that I would pay for the tab of meat eaters.
Sure, I'd prefer they not order meat, but these are family and friends whom I love. I feel the gesture isn't about what they order... it's more meaningful than that. My love for my grandfather and returning the gesture meant more to me than what he ordered. I do the same with others. I don't wish to make my loved ones feel judged or bad when we get together.
I'm not thrilled to be paying for meat, but being with my family/friends is more important. I don't want to alienate myself and put them on defense. People who are put on defense close their ears to your message anyway, so it will be lost. I couldn't imagine anyone telling me they would take me out as long as they could choose what I ate. I also think that it depends on how long you have been vegetarian or vegan. In the beginning I was much more rigid with it. Today (20 years later) I'm more understanding and non-judgmental toward those who eat meat. After all, I'm dining out with them because I want to be with them, so I need to accept that they eat meat (even though I'd prefer nobody did). I try to lead by example and celebrate even the small positive changes people make.
If you are someone who likes to dine out with family and friends, then you will probably ask yourself this question at one point or another. I can understand why some vegetarian and vegan people refrain from paying for the meat that others order. I really do. But if that's how one feels it may just be better to avoid dining out with meat eaters, so that the issue never arises. We can't just care about hurting the animals and not care about hurting our family and friends.
Along the lines of this same topic, I did spend a week with my grandparents not long before my elderly grandfather died. I cooked for them while I was there. That's one thing I cannot do is cook meat. I let them know I'd love to make them meals while I was there, but it would all have to be vegetarian. They didn't mind one bit and actually enjoyed the meals that I prepared for them. So, the situation really has to be taken into account when making your decision. I wouldn't pick up the tab for a pig roast, but I would to buy my grandfather breakfast out. You have to assess the situation and determine which will work for you and your family and friends.