part of being a human being is having a physical body which will have features people like and another part of being a human being is having a libido and being interested in other people. the hard barrier you make going from "that person is pretty" (we can rephrase this to be "that person has overall features I consider attractive and possibly I find myself attracted to them") and "I'd fuck her" (that person has overall features I consider attractive and I find myself so attracted to them to the point I would have sex with them if given the opportunity) is in fact not a line going from not objectifiying to objectifiying but a natural continuation of the feeling and logic. the only real difference is the level of respect being given since "I'd fuck her" is a much less respectful phrase and the fact sex is being talked about which is why this ties into "puritan culture"