i definitely have more to add (disclaimer I'm american and mainly talking about america, but many of these will still be relevant outside of that to varrying degrees)
legal incentives for marriage: there are many legal benefits (tax credits, split ownership, medical rights, etc) that can only be gained through marriage, as well as much of society being structured around marriage, making it much more difficult for both single and multiply partnered people
expanding on issues with multiply partnered people, amatonormativity also applies to non monogamy, or poly relationships. the expectation for relationships is two hetero people in a life long monogamous child bearing relationship, and so relationships that exist outside of of that are heavily scrutinized, if they're tolerated at all. This is also where heteronormativity overlaps heavily with amatonormativity!
now expanding on child rearing: the invention of the nuclear family model also erased the use of community raising of children. throughout human history, raising children was really done by the whole community, with varying degrees of blood relatives, neighbors, friends, and etc, and this allowed for children to have a wide support network and be well taken care of and protected from abuse, since a child with more adults to rely on isn't going to be isolated in abuse as much. nowadays, it's very hard for more than 2 people to be legal guardians of a child, and even adopting while unmarried or with more than one partner is near impossible.
expectation to have children. this is another overlap with misogyny, but part of amatonormativity is not only the expectation to get married, but also to have children! luckily more and more people are choosing not to have kids and this is being more widely accepted, but its still a default you have to "opt out" of and have a reason to not have kids, otherwise something is "wrong" with you/your relationship. it is much healthier to only have kids if you actively want them, rather than have them because you don't have a "good enough" reason not to.
Living wages. it takes two adults working full time above minimum wage to provide for a family, making being a single parent much much harder.
expected structure and progression of a relationship. the ideal relationship model is made out to be a monogamous romantic relationship that moves at the right place- too fast you're a slut or needy, too slow you're prude or cold hearted. romance is also expected to involve things like kissing, cuddling, and sex to the point where people often feel like they are owed these in relationships, even if they've never communicated with their partner about it and learned whether or not they're okay with any of those things. this expectation is also particularly hostile towards non romantic life partnerships (which yes they exist and no its not homophobic to be a same sex couple engaging in a non romantic and/or non sexual life partnerships. these are still good queer rep and important to respect and include.)
partners are property. this can overlap with misogyny, but exists outside of that as well imo. a romantic partner is seen as "belonging" to one another, where they may feel entitled to your time, emotions, body (ties into expectations of physical affection and sex), and even your life decisions. this amplified against women obviously, where they're expected to give up personal and professional lives for a male partner, but it can go both ways. imo it's unreasonable to be expected to give up or significantly change life or career goals for a person- your partners should work with your life, not against it. the pressure and self worth tied to relationships leads to a lot of people giving up education, job, or personal fulfillment opportunities to stay in relationships. Hell, I've even know some people where their coming out/transitioning was interrupted or delayed by staying in relationships longer than was healthy, all due to a fear of loneliness.
the idea that being alone = unhappiness, and that loneliness is only solved with romantic connection. people are often told, in exact words, that happiness comes with romantic love. this is expressed in a variety of ways, like that life doesn't start until you're in a relationship or married, or if you break up with someone you'll be unhappy (even if that person is making you miserable), or that a romantic relationship will cure your depression. in reality, none of these garuntee happiness. Yes a relationship has the potential to bring happiness, but that is not inherent to the relationship, but rather a product of the time and bonding spent with another person, which can be done without romance.
relationship hierarchy. this is definitely something I know a lot of aros are salty about, myself included. this is the idea that a romantic relationship is the most important and should be upkept before-or even at the cost of--all others. this often leads to people getting in relationships and just.. leaving behind or dropping contact with friends, sometimes family, and limiting their social connection to primarily one person. this is shitty for many reasons, including the fact that you drop relationships, but also because it socially isolates people and makes them vulnerable to abuse. its expected and normal to drop friends because they don't like your partner, even if they don't like them because they treat you like shit. the hierarchy of romance, then family, then friends last also leads to people not utilizing friends as important social connections even if they're single, which really can amplify feelings of loneliness and isolation. many people keep friends who just do stuff together, but never actually talk, bond, and support each other in the way that's essential for many people.
lastly, and probably the thing that will make people hate me the most, is obsessive shipping. amatonormativity absolutely influences the mentality that characters need to be in a relationship to be happy, and the main benefit of enjoying and creating media is to synthesize romantic and/or sexual relationships. combined with loneliness, hyperfixation or obsession, leads to people who only consume media for shipping purposes, struggle to interpret anything outside of a shipping lense, and will reject or even become hostile towards anything that goes against their ship/s (like people harassing creators to make ships canon, ruining careers of people that don't make ships canon, shipping irl people including minors, "shipping discourse" leading to harassment campaigns, violently rejecting non-romantic interpretations of a relationship, even accusing real life people of "queer baiting" for not being out/in a relationship. it goes on and on). Not saying all shipping is bad or goes that far, but its core it's absolutely saturated with unchecked amatonormativity, and unfortunately most people are unwilling to hear any criticism of fandom, especially when it involves examining its own toxicity. this has definitely led to many aspecs abandoning fandom, or only engaging in it within the aspec community.
and yeah. that's all I got rn, but tldr amatonormativity absolutely exists and affects everyone not just aros, and even significantly shapes society.
