I don't want to get into an argument, but he IS right. I learned this in my biology class in college. The vagina is designed for sexual penetration, and because of that, the inner lining is much sturdier and more durable. The anus, on the other hand, is not, and will bleed under pressure, making it more succeptable to the acceptance/transmission of viruses. Scientifically speaking, the vagina is more durable.