CacackleLasson Because apparently alpha males are hypermasculine chads who sleep with every single woman under the sun, and if you're not an alpha, you're a beta. Also, because many of these readers who throw these insults are most likely hormonal teenagers who have no idea how relationships work in reality, and they are just there to read the sex scenes. So if the protagonist doesn't give them what they are here to read, they get mad.
Tomoyuki Im actually writing what you are describing and I'm somewhat concerned since the name of my MC is Chad. And he has only one love interest and the theme is beta to alpha, Im concerned people might poke me and they will say he is still beta.
HotRedFlaming Alpha isn't about sleeping around, rather it's being attractive to other people (like, all girls want to be with him and all boys want to be him)
I personally find interesting those characters that could have everything, but they choose not to
xiaohai_23 I see, thus far no one is complaining so I might be doing okay.
That not what the hormone rich male teens think. They have another term for that and it call the omega lol..
They just have no idea that in real life, people needs to socialize due to jobs and social circle hehe.
Avidfan can u describe an omega
Avidfan Oh my, are you talking about that thing like Alpha, Beta, Omega from werewolves BL stories?
It makes me shiver because it's like the opposite of the scientific meanings of the words. I seriously can go along with that concept, so I haven't read stories about werewolves just because of that. I can't force myself to open the first chapter after the synopsis.
HotRedFlaming well, in girl's stories, the omega is like the one that is destined to be with the alpha (which is so confusing if you have common sense LOL)
Now I'm curious what is it in boy's stories
According to some, it is the worst mc...
Alphas are the ones that lead the pack and the only ones that breed, Omegas are the ones that are at the bottom of the hierarchy completely, and betas are the ones in between. A protagonist will be called omega when he's pushed around more than usual.
You are prolly a girl...cos these are the internet slangs used by the males. Tomoyuki above has explained very well =)
Avidfan Ah, ok. This makes more sense.
It'll help me forget about the ABO from erotic novels and the likes.
Avidfan ok, thanks. I should have figured it out sooner :D
Honestly most harem stories have plots that leave the reader unsatisfied, angered or betrayed. And thats why most of my friends and i tend to stay clear of the tag.
Dare43564 whats the reason
CacackleLasson Personally, I hate harems.
I'm referring to the types where the ML has a whole load of women that he sleeps with, and be with. Like, he can't keep it in his pants. The women are normally portrayed as beautiful women with a variety of personalities, mostly spoilt, stupid, wilful, etc with a handful of strong women who aren't in their right minds for sharing a man ... imo.
I don't mind if the guy has a whole truckload of women lusting after him, but he only responds to one, is faithful to one, and remains with one. The same one, mind you. I don't even know why some would regard this as a harem.
A harem is basically where the women are all who the guy sleeps with and keeps by his side. They share one man.
It just doesn't vibe with me because I like the fantasy that a man can love and be faithful to one woman, no matter how many fling themselves at him.
I don't mind harem as long as the guy sticks to only one girl and very vocal about his relationship with her. Meaning he wouldn't hesitate to say to anyone that 'hey, this is my gf or she's the girl I like. Etc. Etc.'. I don't like it when guys are being very vague and stringing along the other girls who like him. Which is mostly the case in anime/manga/light novel harems. That's why I don't watch/read those types. It always irritates the heck out of me. 
CacackleLasson Those feeling emerge from the cliche turn of events. Stupidity of the leads and how they only think about themselves. The only think about their issues but they forget the other characters and use them as disposables.
Personally, I don't like harem (be it harem or reverse harem) I prefer the one where the protagonist loyal to the love interest, so it's just one person. (Male and female)
However, I do have read novel that the MC has harem and fine with it since it's not the main focus of the story. The example will be "Wu Dong Qian Kun" and "Warlock of the Magus World"
Both have harem tag, but the story didn't revolve on that point and only ever touched the harem topic after several hundreds, nearly 1000 chapters. (Well, I did skip the chapters when it's about their relationships with their women, though. Since it's not many, I'm not that bothered as the story is good in overall and I read the story mainly because of the plot)
Well, this is just my opinion and personal taste
Peace 
PsyberRose Wait a minute. I know Im not a master who know or understand about harem or even about alpha beta omega delta gamma sigma epsilon etc. But the way this one describe about harem ; MC has lots of women chasing after him, but the MC only have one wife/girlfriend/one woman only that the MC choose and being loyal too, and the other women still try to get together with the MC (lots of pelakor?) even saying they dont mind sharing the MC with other women, but the MC still with his decision with only one girl and the decline the others softly/politely, and then the readers also really like the other girls and also really wants them to be with the MC, but since the MC did not, they would call him a Beta MC?
Because an Alpha male, usually on a pack (wolf pack? Or any other pack?), have to take care (breed? Mating?) with lots of female under them? So if MC choose only one, he is not Alpha, but Beta? And since lots of female are already introduced, its Harem?
Hmmm.... this is reminding me of some old harem manga; "I'S" and "Ichigo 100%" and "Love Hina" and some other too.
Also reminds me of some webnovel ; "Release that Witch" and "The Novel Extra" where I also really-really-really wants them to be harem, but they are not harem. And they have lots of lovable different kinds of female characters there.
Why? Why! WHYYYYYYY!!!!!! 
NAZEDAAAAA.....
Ahhh, I want to share my opinion but I'm not a woman xD
But when I talked to many of my females' friends, their opinion is that woman/man is disgusting . "not only want him/her but the other too".Bite off More Than You Can Chew.I Just basically share their thought.
