I don't think objectifying people is the same thing as assessing them. I think assessment is more about how we fit into the environment around us.
Assessment is done by acknowledging what clues the outer appearance of an individual tells us. It requires us to assess ourselves in the process. I'm sorry but you can't tell me a 6 ft tall woman is always looking at a 5'4" man and considering him a threat just because he is a man. I'm short so a 6 ft tall women could be considered a threat to me and a guy my size won't be seen as a threat to me.
I walk into a meeting room and realize I'm the only woman. I'm not going to consider who might attack me but I might take note of who is attractive. I walk into an alley behind a club for a smoke and realize I'm the only woman - I'm going to be on guard and not very concerned with who is attractive or not. If that same alley was full of nothing but women considerable larger than myself, I'd be in the same situation.
Objectification reduces the individual to a thing and what uses one might have for that thing.
I've noticed that the men I've known who had the most sexist attitudes were also the men who thought of sex as something they did to a woman rather than something they did with a woman. There was always this vibe of women being sexual victims rather than participants. Sex being something they had to convince women to allow more than it being something women would naturally want. I also notice it with the threesome thing. How often it is read "we met a woman and had a threesome and it went bad after that"? Yet she wasn't seen as a potential threat prior. Rarely so nonchalant in the choosing of a male addition.