First I want to state that I am talking about biological sex differences in human anatomy, So I am referring to gender male/female in a purely biological sense. Even those who identify as a different gender (Gender Dysphoria) show these same brain structures even though they identify differently.
Second if a pathologist found a disembodied brain, could they identify it as a male or female?
The answer s actually no. They could measure the size, the grey matter/vs white matter but they could not say 100% if the brain was male or female, they could say it was most likely a man's brain or a woman's brain but 100% certainty no they could not. But in general a women's brain has more white matter, and a man's brain has more grey matter (source) "Gray matter represents information processing centers in the brain, and white matter represents the networking of – or connections between – these processing centers." See this article for more information.
But, if the brains were functioning and connected to an fMRI machine it would be quickly obvious whose is whose and which is which. As an example here's a pair of videos showing the difference between male and female orgasm.
And there are reasons for this: The cognitive differences between men and women. Stanford University.
Now to the point of my article. One of the things that I wish women understood about male arousal. The section of a man's brain that is stimulated during arousal is his senses. Especially his centers for sight. A guy literally sees sexy. While a woman thinks sexy. Study showing the differences in male and female arousal.
A woman can see something that she finds sexy such as:
But the process is like this. Image, then thought about the image, then sexual response. While in guys that middle step that thought is not there.
So a guy sees a sexy image:
It goes image, sexual response, then thought..... So when we talk about sexually objectifying women there is an assumption that guys have a step to stop that response. They do not. Even a guy having sexual fantasies triggers his sense response (imagination of images) rather then a direct sexual thought.
This is a profound difference in responses. So next time your guy sees something attractive, understand that they can not help it. The sight alone was the trigger, not thoughts behind it. Of course a guy can then think, and react appropriately, turn his head, etc. But that initial response is hardwired, and there is nothing that he can do to stop it.