Why do men make it so obvious when they stare at that area?

I was eating at a place in a spot where there were no other people because it was so quiet due to the holidays.
When I sat down, I took off my jacket. A staff member came over to empty the trash, and it was obvious he was staring at you know where. He didn’t even look away when I noticed it. His eyes got big, and he just stood there for a few minutes.
Why do men make it so obvious when they stare at that area?
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