TLDR: Pedophilia is seen as wrong for a few reasons:
- Radical feminists view men's attraction to women as a threat to their saftey. Ergo, they see men's attraction to children as a threat to their saftey.
- People project the predatory and amoral way adults handle sex with each other onto a pedophile's interaction with children. They don't view children as having the agency or knowledge to assert themselves against predatory and manipulative adults like women do.
- Feeling as though the morals of society are being eroded away and that we are becoming more like animals.
- Hostility towards deviant sex legitimizes "normal" adult promiscuity.
- Keeping children in a traditional innocent role makes people feel like they have morals.
Women used to be seen as innocent, pure, naive, and vulnerable to the predatory desires of men. In order to protect a woman's virginity (which was seen as essential to their value), a woman was to only have sex within marriage.
Over the past two centuries there has been a pushback away from traditional values. Marriage is no longer required for sex. Women are given more agency and are seen as equal to men in society. Because of this, women are now seen to have the agency to "stand up to" and "defend themselves against" men's predatory behavior.
In addition to this laxing of traditional values, society now permits all kinds of behaviors under the notion that consent is all that is needed for something to be considered morally permissible. Now that sex is no longer tied to marriage, it permits a wide range of behaviors that were condemned in the past such as promiscuity, prostitution, incest, homosexuality, pornography, etc.
Consent-based morality is also problematic as it essentially allows women and men to behave like sociopaths. If the only thing defining consent is permission with a lack of force, pressure or coercion, then that allows people to use underhanded tactics to gain sex such as lies, manipulation and exploitation. It allows people to take advantage of others that are more naive and easy to manipulate. Women still see men's behavior as predatory and manipulative, but they are now seen to have the agency to deal with it. Women are no longer seen as naive and vulnerable - they now have their "guards up" when dealing with men.
Society subconsciously feels like they are heading down the path of moral decay, which is a remnant of puritanical guilt from earlier in our history. In response to this, society has become increasingly protective over children's "innocence" - attempting to protect them from the sin of the world, which they view as sex. Children are raised under a puritanical assumption that they are meant to be virgins until they are wed, at which point they will have sex only for procreation - just like women 100 years ago. Except nowadays, it is socially unacceptable to push traditional expectations on young adults, so the minute a child turns 18 parents avert their eyes, look away, and go "you're an adult now, you can do what you want." But those expectations will reappear at some point.
This allows people in society to feel like they still have moral standards. They may not have those standards for
themselves, but putting them on their underage children makes people feel good about themselves. Anything that exposes children to sex as being something pleasurable is seen as "corrupting" them - because it sets them off the path of purity and toward a life of senseless hedonism, just like adults in society.
Young adults are seen as older children in a sense. Since they are above the legal age, people expect them to live free and wild, but then eventually fall in love with someone around the same age and get married. An older adult getting into a relationship with a young adult is stereotyped as taking them out of a carefree lifestyle and shackling them into a traditional role. Since again, people do not want young adults to have traditional responsibilities and expectations while they are young, this upsets many people.
Not only does society want to protect children from sex, they want to protect them from
sexualization which can be as little as thoughts about children in a sexual light. This is why people in society are not only hostile towards those who show an attraction to children, but to those who they suspect show an attraction to youth itself. So even children and teenagers can be called pedophiles if their sexual interest is younger than normal. This would also include being attracted to young adults. Since children and young adults share youth in common - many paranoid people feel as though if an older person finds a young adult attractive, that attraction could somehow translate towards little kids.
They always say "men would go lower if they could" in regards to dating an 18 year old. This is true of course, but they fail to take into account that it's not just youth that men are interested it. It's a biological fact that men find post-pubescent features attractive. So while the age of attraction for most men would go lower, it's not like it would go further down into prepubescent age groups. People who shame men's sexuality like this are always careful to not mention this fact, because they don't want to lend legitimacy to the biological basis for attraction.
This faux concern over young adults disappears when they reach an age where they are no longer considered youthful looking - around 25. That's when all the concern over them being naive and dumb disappears, and they can now date people decades older than them. It's not that they have somehow attained new maturity or cognitive abilities at the age, it's because since they are no longer considered young and so attraction to them doesn't imply an attraction to youth.