Keep it simple. Just apologize and throw it away to keep the peace. You may or may not wish to add that you didn't realize that that type of thing would offend her and remind her that she is the most beautiful and sexy woman in the world to you (hopefully you do feel that way!). Personally, I think it's kind of a silly thing to get upset over (but absolutely do NOT say that!), but obviously she is insecure and threatened by the images in the calendar and needs validation and reassurance. Give her what she needs.
Men are aloud to look as long as they don't touch a calendar is perfectly heathy tell her to calm down my man had a bunch of naked playboy clocks and he collects them. She needs to work on herself a little
It could be worse....atleast they were wearing clothing... as others have said don't lie but make sure you hit the subject that you don't favor any of those women over your wife. If shes pissed its because she has self confidence issues (trust me I'm the same way :p)