One in the pink two in the stink?

So for the past few week my girlfriend Sandra and I have been pondering ANAL sex. I know the dangers of Satan coming through her butthole and stealing my man genitals are very very likely, and I do not know what to do. We both want to have sex with her anus, but I do not want my penis to be thrown into a never ending void. Is there anything i can do to prevent this. I am severely allergic to all types of condoms and I like it when its dirty, Please help I don't want to lose my little willy.