how safe is heterosexual anal sex if neither partner has a disease
Hi. Thank you for your questions. Many people incorrectly believe that just having anal intercourse will give you a disease like HIV/AIDS. The fact is, anal sex by itself is not what gives you an infectious disease. It's unprotected sex WITH AN INFECTED PERSON that makes intercourse risky, as far as HIV and other Sexually Transmitted Diseases (STD's) are concerned. If you have unprotected anal sex with an uninfected person, you are at no risk whatsoever for HIV and other STD's. But, if you had unprotected anal sex with a person who is infected with HIV or other STD's, then you would be at high risk of infection. This is true whether you are the insertive partner (the "top"), or the receptive partner (the "bottom"). If there is any possibility at all that your partner may have HIV or another STD, I strongly encourage the use of condoms every time you have sex. STD's that you can get from anal sex include HIV, Hepatitis B, syphilis, Herpes, genital/anal warts, and others.
Besides STD's, anal sex can have other health concerns as well. There can be physical trauma to either partner during anal sex. This is because there is a lot of friction that occurs during this activity. The receptive partner (the "bottom") is at particular risk of physical trauma, especially as it relates to bleeding. One way of reducing (but not eliminating) the problems of physical trauma and bleeding during anal sex, is to use a lot of lubrication during anal sex. If either partner finds anal sex too painful or uncomfortable, they may choose not to engage in this activity.
Some people can also have physical reactions to semen during anal sex. For more information about this issue, see the question, "Anal Sex".
In summary, as long as both partners do not have any Sexually Transmitted Diseases, and as long as both partners do not have any other problems regarding anal sex (severe bleeding, pain etc.), anal sex is a safe option for consenting partners.
If you have any further questions, please feel free to call the Centers for Disease Control at 1.800.232.4636 (Nationwide).