Nature arguments
Nothing can change female sexual desire (biological essentialism) in a free mating market, post-sexual revolution
SMV is determined by things men cannot easily improve on such as looks, money, status, physical stature, race, IQ, and neurotypicality. It is argued that women don't care about a man's personality, which is often conflated with morality. Some argue that only looks matter. Some say stuff like 'confidence' etc. matters a little but much less than readily observable material factors. SMV is argued to be the same as dating success, and it is commonly argued that direct female mate choice is primary in determining a man's sexual success, and not other external factors, apart from rare circumstances
Cosmetic surgery can drastically improve men's mating success
That men's genetic quality is accurately determined by women at large (good genes hypothesis) and that this applies today; therefore incels have 'bad genes'
Women like big penises
Women care greatly about a male partner's height
Forced sex is not so bad and many women like it, at least if the rapist is an attractive and/or high-status man. Popular BDSM themed porn aimed at women like "Fifty Shades of Grey" and the like are often cited in support of this position
Women are hybristophilic and attracted to male dominance and displays of violence in general
Women bad
Women are only capable of love in case of high status men. Basically that women can only love conditionally, which most wouldn't consider 'love'
Women reject 80% of the male population, or would if they could perfectly optimize their mate choices
Women will reject a man because he is a virgin, unless he is very attractive
Women mostly have a horrible character, if any
Women take joy in bullying at least as much as men
Women only love what men can provide for them, financially or sexually
Women's depression and loneliness are at best self-inflicted, at worst a means of getting attention. It is generally argued that women suffer less than men, as they live life on 'easy mode'
Women love playing victims to get their way
Women are sneaky and lie a lot
Women behave like children
Women exhibit a dual mating strategy, where men who financially provide for them are often cuckolded by more masculine or attractive men
Women have a passive, transactional and opportunistic love style
Women are solipsistic
Women are particularly shallow about the looks of men, and care as much, or more about looks in a romantic partner than men do
Women's sexuality is more primitive than men's, who are argued to be the romantic gender, but they conceal this to receive investment from male partners. Some argue that they are stuck r-selected past as they have been subject to less selection pressure
Society
Men need sexual intimacy and romantic intimacy
Inceldom is harmful
Making women non-financially dependent on men for survival reduces them to their looks
Polygamy in general is bad and always unworkable on a community level, as it creates incel men who are excluded from reproduction and sexuality
Eliminating traditional gender roles destabilizes society through creating demotivated male incels, as women are claimed to 'ride the cock carousel' and engage in de facto polygynous arrangements with 'Chads' in contexts where they are free to select males and defer marriage
The patriarchy would solve or mitigate inceldom to a substantial degree
Wife sharing and other forms of extreme promiscuity are pathetic and necessarily chaotic
Religions were right about women
Rich men are often merely betabuxed
Women are compatible with forced marriage as they naturally fantasize about sexual coercion
Sexual coercion is a natural part of human sexuality
Revoking women's right to vote would be a good idea
Many of these themes can be found in incel vocabulary and memes. Looks and status being necessary for men, but unevenly distributed, gives rise to beta and alpha males, Virgin vs Chad. Chad represents women's tendency to date up (hypergamy) and to prefer dominant men. The higher sex drive and more active love style in men give rise to beta orbiting. For women, Becky vs Stacy represent women's intra-sexual competition in looks.
Blackpills often means that "it's over" for incels with poor looks and low social status or other flaws—that is, that they have next to no chance of 'ascending' or attaining sexual and social fulfilment. The blackpill gave rise to various spin-offs with varying degrees of seriousness, for instance, the 'dogpill' or 'birdpill'.