Most people consider it un-natural, due to lack of understanding, thus those people may not accept aro/ace people. It's considered un-natural because sex and romantic relationships advertised and is ingrained into our minds because it's everywhere. It's instinctual to have sex, and most of us start by having a purely romantic relationship which later turns intimate. From a young age we are all taught (especially women, but that's a separate issue), consciously and sub-consciously, that part of our purpose in life is to have children. Obviously in order to have children an egg must be fertilized by a sperm during sex but if you're ace/aro, depending on what you are comfortable with, this won't happen; when this doesn't happen you are not fulfilling your "purpose."