Though homophobia and heteronormativity are linked, it's not the same. Homophobia drives everything you've written, heteronormativity drives the basic assumptions about one's sexuality

The problem your having isn't with any normativity, it's with tribalism. There’s an awful part of being human that makes us want to be part of a social group and shun the outsider. To decide that those who aren’t with or like us in our beliefs and norms are evil and bad and to be hated. And even worse, when we feel socially and morally justified in hating someone it feels good to vent that anger and hate at them.

It's two combined evolutionary relics from millions of years of what eventually became humanity. First, through almost all of that time the outsider was dangerous. Food was often scarce, survival was always questionable, and medical care didn't exist. A group of people you didn't know could easily become a fight for resources, and even a single clearly friendly unarmed stranger could bring disease and wipe your entire group out.

The second is the social pressure of fitting in. Through almost all of that time being kicked out of your group was likely a death sentence. We can't seem to shake that inner terror we're rarely even conscious of about not fitting in. It pushes us to agree with the people we want to be close to or be liked by and to try to emulate the group we're with.

Combined, those things are awful. People anywhere will always notice someone who seems dramatically different from the area norm, but someone can stand out in a rural region when the only distinction is a slightly different accent. The drive to shun the outsider and to drive to fit in with your known group work together to make it likely that whatever the difference is, the group is likely to treat that different person or group terribly.

Once someone begins it and no one steps up to stop it, it starts to feel acceptable and even right to do it. Pressure to fit in becomes pressure to join in, and the smaller and tighter knit the group is the more difficult it is to deny that and deviate from what now seems to be the norm of our group.

If you hear someone else from your nation make a racist comment to an immigrant it's not too difficult to step in and say something against it. If they respond back with some angry tirade about how you're siding with "them" there are good odds you'll leave it at that and move on, but perhaps you'd become adamant and insist it stop.

On the other end of the spectrum, if you're spending a day with your closest friends and one of them makes a racist comment to an immigrant it's much harder to deal with, and even more so if it's said privately between the group or other members of the group looked entertained or uncaring when it happened. It's so hard to even bring it up that people typically decide to let it go or debate whether to say anything internally. That pressure makes it really easy to join in with those acts, even when they're things you really believed were wrong.

The world has over 8 billion people spread among something like 17,000 distinct ethnic groups and cultures, all with varying histories, religions, moralities, and social norms. Aspects of normal behavior to one group is very often considered immoral to another, and at times there are complete contradictions to the point where doing something is immoral and illegal in one place but not doing that same thing is immoral and illegal in another.

When someone sees your beliefs or acts as immoral it’s easy to define that as them being hateful of you. If it's a group of people it's easy for them to become hateful of you. And because of that, you're likely to hate them in return. And you both spew hate at one another, and you both feel good doing it, because you can both tell yourselves that you're justified and the other person is in the wrong.

You'll never beat normativity. Macrosociology and cultural evolution show that moral norms such as monogamy or polyamory fail to unify across a population once it grows into the millions of people. And our actual social strata doesn't extend nearly that far in reality. The highest number of people typically considered able to maintain a cohesive cultural group before they naturally begin to differentiate is only 150. Since people constantly grow and change the norms of any group and culture are always in flux, they're always going to be there and always changing.

The best solution is simply doing our best to understand that people who have completely different beliefs and morals than us aren't evil. That it's great to do our best to live up to our own morals and values, but that it's not right for us to try to force them onto others. It sounds easy, but in also means not judging others based on our own morals.... so it's extremely difficult.

There are people in the world who consider dog meat part of a normal winter meal. It's very hard for me to not consider them evil, horrible people. Dogs are furry bundles of pure love who want nothing other than to be near their person and enjoy life together. They don't see them that way. Seeing that as a cultural difference and not them being evil is tough.

That's just pets. It's way harder when it's something deeply personal to our own life and self. Your example is clearly one of those. It's likely the biggest and most common. Not just the mononormativity and platonic amatonormativity aspects, but the entirety of who it is and is not considered moral to have sex with and under what conditions. It seems very likely that it's one of the things the moral differences that so much of humanity feels so strongly about even though there are many different accepted norms around the world is a third evolutionary relic. Survival of the group/society/culture/species is the primary evolutionary imperative, so it makes sense that the acts associated with human reproduction are the ones that so many people have a hard time accepting norms on that are completely unlike their own.

All we can do is try to live up to our own personal beliefs and morals, and struggle to not see those whose norms and morals don't align with ours as evil. Maybe one day we'll all be able to succeed in that. But we can at least understand it and keep it in mind so that we can keep ourselves from acting hateful towards those people and hopefully become more willing to try to stop our closest friends from acting hateful as well.