Who is a rapist? That's a question our societies began to struggle with as far back as when we determined that women had the right to refuse sex.

More often than not, we just say who isn't a rapist. Deuteronomy says he's not a rapist if you didn't scream. Some judicial systems say he's not a rapist if he's your husband, and increasingly few say he's not if he marries you right after. Cops, prosecutors, juries and members of the public who eventually get called to serve on them have myriad of unwritten rules about who isn't a rapist: famous men, men whom you kissed that night, men with whom you (or anyone) previously had consensual sex, men who were going to pay you for it, men who are "too attractive" to have to resort to coercion, men who didn't have weapons, men who haven't ever raped anyone else and seem too old to start now … and the list goes on.

But this week, a user on the social media site Reddit asked fellow users to share their experiences as perpetrators of sexual assault. The end result was part mass-confessional and part exhibitionist spectacle in which some users offered one another virtual absolution in the same way society often already does.

But in between the remorseful paeans and the sociopathic how-to guides, an existential truth emerged: many rapists look and act like everyone else, almost right up until the moment they start raping. It's a truth that many sexual assault survivors have already been forced to confront, and one at least as many survivors know the rest of society hasn't yet wrapped its head around.

For all that we've moved modestly beyond the idea that a rapist is the transient in a back alley or the creepy guy eyeing your unattended drink in a bar or the angry, sexually frustrated predator, we still believe that we can tell who the bad people are. We believe that rapists are defined by their criminality and that people whose lives aren't defined by that criminality – be they sports stars, coaches, international leaders, whistleblowers redefining journalism in the digital age, the creators of popular movies or songs, or just the nice fatherly guy next door – well, they can't be rapists.

Life, and people, are more complicated than that. The cute guy who let you cuddle up next to him during a movie becomes the man who crudely forces your legs apart and enjoys himself more when you struggle. The friend who you let stay in your extra bed becomes the man who shoves his fingers in you when you're too drunk to know.

The hook-up buddy becomes the man who ignores your boundaries and your nos and forces open your clamped legs. The guy in your group of friends who you were laughing with half an hour ago becomes the man who walks into your room and molests you when you're asleep, even turning on the light to inspect his work. The older guy who was nice to you in the afternoon becomes the man who shows up in the middle of the night and pins you down in your own bed with his body. Your loving boyfriend with whom you consent to sex becomes a man who clamps his hand over your mouth to stop you from screaming as he anally rapes you while in a "trance".

Some of them stop when you cry, when they look at your face and see you feel you're about to be raped, when you say no, when you push at them screaming. Others don't. Some go to jail, but most of them don't.

Some grow up and marry women who don't know what they did, have children they want to protect from men like them, smile at you at PTA meetings, or even show up at your door years later to look over some storm damage on behalf of your insurance company. Some stop, some learn better, some spend years hating themselves; others brag about it, or self-pleasure while remembering it. But one thing they all have in common: none of them looked or acted like rapists, and few in their lives, unless they were reported, suspects they were.

In many cases, they aren't the violent sexual predators you've been taught to expect and guard against. They aren't always sadists who enjoy it more because it hurts (though they exist, too), but, at some point, they all just stop caring how you feel because that isn't remotely necessary for it to feel good to them. In some way, they're reading off the age-old script: sex is something you have that they want, and your resistance is just a barrier to push past– or the lack of resistance, even if you're incapacitated, is acquiescence.

If there's one other lesson to be taken away from the morass of that discussion, it is that there's no amount of warning women not to leave the metaphorical keys on the hoods of our cars that's going to end rape. For every man who admitted to plying victims with alcohol, another simply held a woman down; for every man who professed to being so aroused by a woman that he "had" to force her, another admitted to molesting a woman who was unconscious; for every man that went after a near-stranger, another hurt someone to whom he'd been close.

The people who need to be educated about rape are our men and boys. They need to learn that sex isn't a zero-sum game, it's not keep-away or capture-the-flag, it's not a thing they do with their genitals to the genitals of another person at whom they don't look, let alone see.

It is something they can engage in and share with another person and, if the other person – the whole of other person – isn't sharing in the sexual act, for whatever reason, at whatever moment, then it's not sex. That's when it becomes rape – and no matter what you look like, or what other good you've done in your life, you're then a rapist.

And, if the men who admitted to it on Reddit are any guide, no matter how they justified it in the moment, they know it.