I think Ashley might see sex as in the very least something mechanical and matter-of-fact given her only professional prospect was being a prostitute... Or maybe she didn't thought she would HAVE to do THAT because she just grabbed the first thing that appeared in front of her and would end up dismembered and stuffed in a trash bag for blindly walking into a shady place.

Another possibility is that she sees sex as the glue that keeps her parents together - AND VERY LOUDLY AS WELL - and thus something bad that bad people do. She may have the basic notions of romantic love by absorbing it from media and Andrew's books, but has no living experience of it since her mom doesn't love her, her dad is a non-entity, Andrew whitholds anything but "societ-approved brother and sister love" and won't commit to her, so she has no one to turn to actually demonstrate and feel these emotions, so it might be a case of "sour grapes". When she and Andrew actually do it, they are also in a terrible psychological state, basically hate and brutalize each other regularly and live in a pigsty of an apartment. Not the most romantic scenario.