My wife is pregnant with twins, and her doctor will only do C-sections for twins. Can't she have the babies vaginally?
There are only certain situations in which twins may be delivered vaginally: first baby vertex (head first); second baby either vertex or breech. (If the second baby is breech, the baby can either be allowed to deliver vaginally if all of the other qualifications of vaginal breech delivery are met; or if the prospective parents agree to a breech extraction, in which the baby's legs are pulled down to begin the breech delivery.)
Breech first is a problem, because it's a high-risk delivery that jeopardizes both babies. And breech first, vertex second is extremely dangerous, because the heads may interlock, which is a real horror story that could lose both babies.
Now about your doctor. . . . Some doctors don't want to take any chances with two babies. Having twins is an incredible gift and there's just too much importance there. Other doctors feel comfortable with a vaginal delivery with the above stipulations. Others have had worrisome experiences.
On my last vaginal delivery for twins, the first vertex baby delivered fine, but then the second vertex baby had a prolapsed cord compressed by his head. It became an "ER-grade, TV-type" emergency, with all of the melodrama and an emergency C-section that delivered, thankfully, another fine baby. Although vaginal delivery, under the right circumstances, is legitimate by standards of the American College of Obstetricians and Gynecologists, and even recommended by many doctors, I think I have a soft spot in my heart for your wife's doctor.