My daughter is vegetarian, my husband is pescatarian and my son and I are omnivorous*. It really isn't a problem when we all eat together. I usually do a veggie meal for everyone, as then nobody is being denied a course or component. There are countless vegetarian cookbooks on the market, and recipes available online if you are stuck for inspiration.
When we all lived together I would sometimes do a hybrid meal, eg a bean or vegetable casserole which I would split into two bowls and add chorizo or chicken to one of them.
Alternatively, you could do a tapas-style meal, so that there is something for everyone and nobody is left out.
*I think this is the bit to hang onto. Omnivores can eat non-meat based meals, whereas vegetarians and vegans can't eat meat. It's not about being difficult - it's just that there is really no room for compromise.