If I were to speak up, in all likelihood my friends would be embarrassed or annoyed, or they might respond by saying, “Okay, I'll buy my own lunch then.” I imagine this turning into a very awkward and difficult situation for me. Is there a graceful way to handle this?
True gifts are those that take into consideration the needs and desires of the receivers, not what the givers prefer. If gifts can't be given from the heart, with no strings attached, they really aren't gifts after all -- they're a way for the givers to gratify themselves rather than the recipients.
When you take friends to lunch, they should be able to choose whatever they want from the menu. When such a proposition is made to meat-eaters, you know up-front that there is a good chance they will order items that contain animal products. It's discourteous and inconsiderate to put restrictions on a proposal to pay for a meal by stipulating which dishes are and aren’t acceptable when you aren't the individual who will be eating them. You can offer to take your friends to a strictly vegan restaurant, but be prepared for them to decline or suggest an establishment that serves both veg and non-veg food. However, if you won't be at peace paying for a meat-based meal, you might be better off not extending such an invitation in the first place,.