Is it possible that in an effort to feel in control of life this article over simplifies?
Other commentors have mentioned the age related aspect - i.e. that older women will more likely have slept with more men. After all, once you've dated a guy a couple of months you tend to have sex since you want to know if you're compatible as well as just being attracted to someone. The numbers would mount up over years, unless you're lucky enough to find your perfect partner 1st or 2nd time round. Maybe we're all faced with too much choice and options these days though, and are less likely to stick around to make something work.
Also, speaking from my own experience and motivations, those so called number 3 types (the hurt and bitter ones) also react that way if they've met someone who's been misleading. They may not behave angry etc when in the actual relationship, but only after when they've genuinely been let down or over played.
This blog alone is testament to the sheer amount of bullshit men will invest in order to get women into bed! If she had expectations she was honest about and he lets her down, she's absolutely entitled to feel angry due to being misled. It's a healthy reaction, as long as she can move past it emotionally.
It can be very difficult to know how a man will react post-sex, even if you've had a talk about not being into anything casual, and been honest about past hurts and that this has made you cautious about getting involved too soon. Some men go out of their way to make you feel safe and set up what you might call 'the nest' to reassure. Then they suddenly realise they're in real feelings territory post sex, i.e. they realise it's no longer a game, that they've been intimate and that this raises expectations in a woman. She doesn't need to say anything, it's just the way it usually is. Some men can man up at this time, whether that means being honest that they made a mistake because they're an asshole, or reassuring a woman so she relaxes into early relationship mode again (you've discussed the post sex follow up care really well elsewhere, but there are surprisingly few men who'll do this unless they're very much in love).
Lastly, I've seen quite a few women, whether in their 30s up to early 50s, who've been bitter and angry or cynical about male behaviour post-sex, but this has taught them to choose better and they're now in long term good relationships or married. Those women have maybe had 20+ lovers if they're above 45 or whatever. I've seen those women bag really great, attractive and experienced men. Experience can also hone discernment, and 20+ lovers does not by any means always indicate either bitterness, or a tendency to not equate love with sex.
These observations are from my own perspective as a woman in her late 40s, my pool of observation is wider than yours perhaps. Also, it must be said, though I've had 20+ lovers, I have never cheated. I do not agree at all with that statistic. Sleeping with several men can indicate many things - i.e. you're addicted to sex, or you're seeking the right man. When you find him, why cheat?! No, my understanding is that in this life men or women are the type who cheat, or not. It's a deeper character-related thing. Still, there are some good observations here, and the 'horny women' feature made me laugh