I don't think women understand this, but men do at a primal, subconscious, genetic level. If a girl hangs out with a guy for x amount of time, and another guy for the same or less time than that, and has sex with the latter but not the former, that's a mortal insult to the former.
The amount of intimacy a girl shares with a boy is the only concrete, objective barometer of their feelings, regardless of what they say. As notoriously fickle liars as women are, only the goods are in the least interesting as a real look into a woman's heart. No matter what a woman says, if she's having sex with y instead of you, she doesn't respect you and doesn't care about you in the least, except maybe as a tool to be exploited.
The more slutty a woman becomes, the more insulting she is towards those she has not yet showered affection upon. The less exclusive she is, the more pathetic you are for not getting any from her. So long as a girl maintains her purity until the marriage bed, nobody will complain about her conduct or treatment towards you, because it just shows what high standards she has, not how low on the totem pole you are.
But in the modern world, where women are often having one night stands with random people they just met, any guy who has been treated as a 'friend' by them for years may as well be an emasculated eunuch, so humiliating is their position vis a vis this slut.
To make matters worse, often enough, the unlucky guy didn't lose out to a rock star or a doctor, but another guy who travels in the exact same social circles as you, quite often one of your own friends. So when you compare resumes you're just left wondering, wait, what just happened, why him but not me? What did I do wrong?
This scorching, seething insult, that somehow everyone else qualified for vagina but you didn't, distorts, preempts, and destroys whatever bond a boy and a girl shared heretofore. There's nothing a girl can say or do to change the elephant in the room that you, and you alone, were not deemed worthy.
This, and not male lust, is the reason why men and women can't be friends. Men can be perfect gentlemen around married women or unsullied virgins. Their conduct makes sense and is totally unoffensive. If you are just beginning to get to know a girl you of course can just be friends, it would be presumptuous to think a girl could quickly make up her mind about such an important subject as who will be her lifelong partner and father of her children. If male lust were so uncontrollable that they couldn't stand to be around a girl they can't have sex with, none of these relationships could exist. But male pride is another issue. You cannot stay a man and lose your pride, your dignity. Once a woman has decided to trample over that pride and humiliate you in the most insulting manner, if you don't walk away from that relationship and never look back you're a cuck, you're a eunuch, you're no longer a man at all. This is why men and women can't be friends. Because once you've given a mortal insult to someone's honor, the idea of friendship continuing as normal is laughable. If two male friends exchanged insults of that magnitude to each other, they would never talk to each again either. It's the exact same thing.
What about the reverse case scenario? If a guy has been hanging out with a girl for years, but only has sex with a new girl he met over coffee last week despite all the original girls' offers? Such a situation is wildly implausible. Why would a guy hang out with a girl for years he isn't sexually attracted to? He'd be far better served with a male friend than an unattractive female friend, who he has much more in common with, and there's no sexual ickiness to worry about. But yes, if somehow the reverse situation came about, then yes, the girl's pride as a woman has been trampled over. All that time and you weren't attractive enough to seduce him, but any old tramp can just walk in and take over at will? Yes, any girl who 'stayed friends' with a guy like that is a pitiable loser you can only roll your eyes at. Get the picture, hag, you're dog ugly and he's never going to look your way even if you wag your tail for a decade.
It's the same for a guy who is dumped. If a girl is willing to break your heart and abandon you, despite everything you did for her, despite dedicating your life to her and all the time and energy you had to give, then you're nothing to her. You're dogshit, and she just treated you like dogshit. It is absolutely unequivocal what just went down, exactly what she thinks of you, by cutting you off in terms of intimacy like that. A dumped guy she wants to downgrade back to 'friends' level has just been insulted even worse than the guy who never got with her in the first place. That guy hadn't yet made the supreme effort to be loved, so he can just assume the girl didn't know what she was missing. The dumped guy gave it his all and still came up short. So short that she didn't think twice before ruining his life, perhaps even driving him to suicide. Can you, with dignity, with pride, still value yourself while associating with someone who clearly values you so little? Can you really come crawling and licking back to her for a few tossed bones after she's already cut you off?
If a girl doesn't value you enough that's on her. But if you accept the low value she has placed on you, by continuing to hang around her like a loyal dog hoping for any little treat, after she's already designated you as discarded or rejected trash, that's on you. And a guy who acts like that certainly won't turn a woman's heart in his favor, it will only make her despise you and mistreat you all the more. Steadily downward dipping manliness, as the woman realizes she can do anything to you and you'll just lick it all up and ask for more, is not the key to a woman's heart.