How can I keep butterflies OUT of my greenhouse?

Obviously, i need to have the windows and door open in hot weather, but do i get an old net curtain accross the door frame? It’s going to look silly!!

I know that many people, myself included love to have butterflies in the garden, but i would just like them to stay out of the greenhouse! They are laying eggs on my crops, and then the resulting caterpillars are eating them! I HATE hand picking them off the leaves - I hate the feel of their wee soft bodies!! Ugh! This IS a serious question!

Question posted courtesy of: Greenhouse Kit Build It Yourself