Here's a "decent" man, though I have no proof of that.
Men's sexuality is difficult for decent men to deal with, especially in a society that displays sexual images everywhere and gives very easy access to more extreme images and videos. Men are in general very easily aroused by what they see. They also have external genitalia which can be aroused by touch - including by clothing when just going about daily tasks. So men's attention can very easily turn to sexual thoughts. Not every few seconds, but particularly in young men, it can be frequently and at inconvenient times.
So a man who wishes to treat women with respect has to manage his physical and psychological urges, and to redirect his thoughts away from anything that might lead to inappropriate behaviour. There is a difference between appreciating beauty in someone and fantasising about a sexual encounter. There is a difference between looking at someone and thinking "how lovely" and staring at someone lustfully ("the male gaze"), but the boundary is easily crossed. Even for a man who wants to behave well towards women, it's likely that he will cross that boundary sometimes. There is a big biological imperative to "spread one's seed" and it's built in to the extent that most men find most women sexually attractive.
Again, crossing that boundary in fantasy doesn't necessarily mean that it has to be crossed physically. Men who sometimes have "impure thoughts" are mostly not rapists, and nor are they necessarily promiscuous, and they do not necessarily ever have an affair. But I think virtually all men are vulnerable to overstepping their own boundaries, and if they can overstep their own boundaries, there is a risk that they will overstep other people's boundaries. This applies particularly to adolescents who are dealing with new urges, but for many the struggle to be true to our ideals and values continues long after that.
Then you also have men who have not been culturally conditioned to respect women, and in them the boundaries are much weaker. Society often excuses lack of respect for women - "boys will be boys", "sowing wild oats" is condoned, porn is accepted, Trump-style "locker room talk" is not condemned. Society says promiscuity is OK or even good, "sex positivity" is promoted, narcissism is normalised in social media, and so on. Boundaries are easy to weaken, and our "permissive" society has certainly managed to do so. Once boundaries are weak or non-existent, men can persuade themselves that giving their sexual urges free rein is "normal", "everyone does it". As a teenager I knew young men who had effectively no thought for how their behaviour might impact the young women they were sleeping with. For example, pregnancy wasn't their concern. How did they reach adolescence without learning that the consequences of our actions are our responsibility, not just the responsibility of those directly impacted?
I will be surprised if no-one reading this thinks I am excusing men. I am really not interested in doing so. Men can and should control their instincts and urges sufficiently to avoid damaging women. It is our responsibility to do so, and clearly at a societal level and at a personal level men are failing to do that.