Yeah, I would guess that anal is safer than vaginal sex, but still fairly risky. (Of course, ask the surgeon!) Some women have more pelvic space and the vagina isn't placed dangerously close to the back passage, but there's no way to know until afterward AND questioning closely if you're one of those, and fistula is not something you want to take chances on. It's pretty much the worst possible complication short of death, as I understand it.