Connect with vegetarian and vegan friends from all over the world.
It may be unnatural to your step-father, but it is certainly not unnatural. For example, the vegetarian Jain religion in India has had adherents for thousands of years, far longer than Christianity has existed. And there are millions of Jains who live long and healthy lives. I was a vegetarian for decades, and have been a vegan for a little over a year. I look ten years younger than I am, and feel it as well. I have not been sick in many years, and I am far healthier than the vast majority of men my age. As long as you pay attention to your nutritional needs, eat a balanced diet and get proper exercise, you can be much healthier as a vegetarian than as a carnivore. Do what you feel is right, and don't let others bring you down.
Whitney, it's great that your spouse and children are supportive and understanding. =) I do believe that you should provide as warm and balanced meals for your family. Invite your step-father over for the weekends and see that being vegetarian doesn't just mean eating lettuce, but have a huge array of food options available. Let him see that your children can grow strong, healthy, and happy from a meat-free diet. Over time, his disagreement will dwindle and he will come to accept that it can be done. Constant arguments regarding a meat-free diet will just put him on the defensive, so rather show him and let him take his time to accept it.